It could also be a major turning point in the war against the extremists. The US-led coalition is carrying out air strikes on Raqqa to support the SDF offensive.
"It started today at dawn", Observatory Director Rami Abdulrahman told Reuters.
The US military has said coalition air strikes in Iraq and Syria had "unintentionally" killed 484 civilians, but observers say the number is much higher. "Our forces entered the city of Raqqa from the eastern district of Al-Meshleb", SDF commander Rojda Felat told Agence France-Presse, adding that clashes were also raging on the city's northern outskirts.
Raqqa is now surrounded from the east, north and west, and opposition activists have reported intense shelling and airstrikes on the city since Monday night, which killed at least 12 people.
IS militants are not expected to give up easily.
The US-backed SDF has been working to encircle Raqqa - estimated to have about 4,000 ISIL fighters - since November, as the terrorist group teeters on the brink of defeat in the Iraqi city of Mosul.
Warning the fight would be "long and hard", Townsend framed the offensive as part of a greater struggle against ISIS, which has claimed attacks in many countries including last month's deadly bombing in Manchester. "ISIS threatens all our nations, not just Iraq and Syria, but in our homelands as well".
"The fight for Raqqa will be long and hard, but the offensive would deliver a decisive blow to the idea of ISIS as a physical caliphate", the coalition said in a statement, citing its commanding general, Lt. Gen. Steve Townsend.
On Monday, the SDF tightened the siege on ISIS in Raqqa city after liberating three more villages in its northern countryside and killing at least 36 ISIS militants.
The SDF has said it will hand control of Raqqa to a civilian council from the city after its capture, echoing the pattern in other areas retaken by the SDF.
Casino gunman identified as problem gambler
Armeen Gomez, chief security officer at Resorts World, said witnesses at the scene had testified to seeing multiple assailants. A previous statement by Albayalde, hours after the attack was over, had said that the suspect was likely to be a foreigner.
The U.S. and its partners train Syrian rebels there to fight IS.
Turkey has remained hostile to the Kurdish People's Protection Units, known by their Kurdish acronym the YPG, which form the backbone of the SDF. Turkey views the YPG as a terrorist group aligned with the PKK which has fought a bloody campaign in southeast Turkey since 1984.
On December 10, the SDF unveils the "second phase" of their offensive, and a Kurdish spokeswoman says the 30,000-strong force has already seized 700 square kilometers (270 square miles) of terrain.
In an address to legislators from his ruling party, Binali Yildirim again criticized the United States for launching the Raqqa offensive with the Kurdish-led Syrian Democratic Forces.
IS stormed across large areas in Syria and Iraq in 2014, declaring a self-styled Islamic caliphate.
The Russian-backed Syrian army and allied militias advanced into Raqqa province from neighbouring Aleppo province where they have seized swathes of territory from Islamic State this year. The fighting has devastated communities in both countries and forced hundreds of thousands of people to flee their homes. It has circled the city with land mines and checkpoints, the AP reported, in addition to "setting up barriers and hanging sheets of cloth over main streets to provide cover from warplanes".
Almost 200,000 people and 5,000 militants live in the city, according to the organization Raqqa is Being Slaughtered Silently. That may indicate that IS "intends to use the 200,000 people still trapped in the city as human shields", the group reported.
State news agency SANA reported on Tuesday that the airstrikes by the US -led coalition killed 12, including women and children.
"The civilians were boarding small boats on the northern bank of the Euphrates River to flee southern neighbourhoods of Raqqa", said SOHR head, Rahman.
French Open Day 9: Murray and Halep hit top gear
Former French Open champion Stan Wawrinka and two runners-up will complete their fourth-round matches on Monday at Roland Garros . Their head-to-head record stands at 2-2, with Garcia winning the pair's only previous meeting on clay in Madrid two years ago.
Former FBI Director James Comey to testify in Senate June 8 People close to Comey have said he wants to clear his name through public testimony. Comey and Trump's working relationship wasn't all bad.
How to watch Mark Zuckerberg's Harvard commencement speech The CEO says that in history, the millennials are one of the most charitable generations. Last week, Zuckerberg posted a video of his reaction when he got into Harvard .
Virender Sehwag forwards two-line resume, BCCI demands a elaborate version
We had to ask him to send his CV too along with his application. "It will the first time he will be appearing for an interview". Amongst all the prominent names that applied for the job, former Australian cricketer Shane Warne was not one of them.
Huddersfield wins soccer's richest game, promoted to EPL Gab Marcotti provides insight into Huddersfield Town manager David Wagner as the Terriers move up to the Premier League . Speaking to Sky Sports , he said: "By the way, Ian Holloway , all the best for next season".
India's Adani gives final approval for $4 billion Australia coal mine
It followed a mining royalties deal struck between Adani and the Queensland state government last week. The company has said it plans to mine 2.3 billion tonnes of coal over the mine's 60 year lifespan.
'Wonder Woman' gross revised up to $103.1 million
Next week, Lucia Aniello, a writer and director from "Broad City", will release her bachelorette comedy " Rough Night ". The studio said Monday that the tickets sold on Sunday turned out to be even higher than it estimated over the weekend.
Djokovic, Muguzura, Nadal in action at French Open
The 14-time major victor has dropped just 20 games in four rounds so far, just one more than he lost at the same stage in 2012. Mladenovic could have company in the quarterfinals, though: Frenchwoman Caroline Garcia plays her fourth-round match Monday.
Harvard rescinds offers to incoming freshmen over hateful memes
Many will agree these students crossed a line and forfeited the right to engage in unfettered debate, at least at Harvard . But with this action, Harvard is sending a message with a classic free-speech chill: You can say anything - but not here.
ICC Champions Trophy 2017
Williamson's departure heralded a mini collapse by the Kiwis as they lost the remaining six wickets over the next six overs. His innings was studded with 8 fours and 3 sixes. "We bowled both sides of the wicket and it was pretty ordinary ".
Semi-finals delayed by light winds The winners of the qualifying series will carry a point into the regatta finale, the Cup Match, if they get that far. The other two semi-finalists will be Artemis Racing and Softbank Team Japan.
Venus Williams exits French Open after Bacsinszky defeat
How old is she? 36? 36? Venus Williams wasn't in the most talkative of moods Sunday after a three-set loss at the French Open . With losses on Sunday by defending champion Garbine Muguruza , seven-time major victor Venus Williams and 2011 U.S.
Australian Woman Stabbed In Throat During London Bridge Attack Our thoughts are with those killed and injured in these horrifying attacks, and we will bring more details as they come in. A GoFundMe page has been set up to support Zelenak's family during their search.
UN Population Fund chief dies
He was married, had five children and several grandchildren. " Funeral Arrangements will be announced by the family ". He had extensive knowledge of the global and national frameworks and processes that are critical to UNFPA's work.
Ex-MMA fighter gets 36 years to life in kidnap, beating case
As Radar previously reported, Mack claimed that her ex-boyfriend walked in on her and a man named Corey Thomas on August 8, 2014. The 35-year-old Jonathan Paul Koppenhaver , who was convicted by a jury in March, will be eligible for parole in 36 years.