A US-backed alliance of Syrian Kurdish and Arab fighters has begun a campaign to expel Islamic State (IS) militants from land north of Raqqa.
The Syrian Democratic Forces (SDF) is believed to have deployed about 30,000 fighters.
US-led coalition warplanes will support the offensive and Russia has also given its backing.
The SDF did not mention any plan to take Raqqa, the de facto capital of the "caliphate" proclaimed by IS in 2014.
The alliance, which is dominated by the Kurdish Popular Protection Units (YPG) militia, has emerged as a key ally of the US-led coalition over the past two years, leading the fight against IS on the ground in northern Syria.
With the help of US airpower, it has taken control of about 26,000 sq km (10,000 sq miles) of territory, including a 400km (250 mile) stretch along the Turkish border.
On Tuesday, SDF fighters were seen moving south from Tal Abyad near the Turkish border towards Ain Issa, a town about 60km (37 miles) north-west of Raqqa, according to UK-based monitoring group the Syrian Observatory for Human Rights.
Clashes were reported nearby, it added.
SDF commander Rojda Felat wrote on Twitter that the goal of the new offensive was to "liberate northern Raqqa" and those living under IS "oppression".
"The campaign is aimed at repelling terrorist attacks on Shaddadi, Tal Abyad and Kobane, ensuring the security of our people," she added.
An SDF source told the Kurdish news agency, Rudaw, that fighters would be "advancing to the villages of Big Fatiseh, Small Fatiseh and Tishi, in order to clear them of [IS] militants first".
BBC Middle East Correspondent Quentin Sommerville says IS fighters are digging in, ready for the assault. As well as defensive placements in Raqqa, they have constructed an extensive network of tunnels, he adds.
A Baghdad-based spokesman for the US-led coalition, Col Steve Warren, said the SDF operation was aimed at "putting pressure on Raqqa", but not taking the city.
The SDF fighters had so far met little resistance, he added.
The anti-IS activist group, Raqqa is Being Slaughtered Silently (RBSS), reported that there had been air strikes on IS positions north of Raqqa on Tuesday, and that intense fighting had erupted around the village of Heisha.
Russian Foreign Minister Sergei Lavrov also said that Moscow was ready to co-ordinate with the SDF and the US on the offensive.
The SDF's announcement followed a meeting between officials in the alliance and a top US commander in northern Syria on Saturday.
Gen Joseph Votel, head of US Central Command, made the secret trip to check on the 200 US military personnel who were advising the SDF on the ground.
The US wants to increase the number of Arab fighters in the SDF, which currently consists of at least 25,000 Kurds and between 5,000 and 6,000 Arabs, before trying to retake Raqqa.
Col Warren said about 3,000 to 5,000 IS fighters were inside Raqqa.
IS seized the city in early 2014, months after it became the first Syrian provincial capital to fall to rebel forces seeking to topple President Bashar al-Assad.
The jihadist group quickly established its headquarters there and began imposing its vision of a state, implementing a strict interpretation of Islamic law.