Fighting between Ahrar al-Sham and the al-Qaida-linked Hayat Tahrir al Sham is the first serious act of violence since both sides reached a truce in February.
Clashes have broken out in northwestern Syria between two of the most powerful rebel groups, raising fears of widespread violence in the rebel-held province of Idlib.
The fighting between the ultraconservative Ahrar al-Sham and the al-Qaida-linked Hayat Tahrir al Sham, is the first serious act of violence since both sides reached a truce in February.
Wider clashes between the two former allies could affect their fight against regime leader Bashar al-Assad's forces who have been gaining ground over the past year under the cover of Russian airstrikes.
Ahrar al-Sham said in a statement on Saturday that the Hayat Tahrir al Sham (HTS) were sending reinforcements to the town of Saraqeb and the Jabal al-Zawiya region in preparation for an attack.
It accused HTS of acts of "tyranny."
The Ibaa News Agency of HTS accused Ahrar al-Sham of setting up checkpoints and detaining one of its commanders and his bodyguard, adding that HTS removed the checkpoints later by force.
Idlib has taken on greater significance in Syria's civil war as opposition fighters and militants moved there from the country's central and northern regions.
Bordering Turkey, Idlib has welcomed thousands of insurgents who left the country's largest city of Aleppo when it fell to Assad's forces in December in the government's biggest victory since the crisis began in March 2011.
Hundreds of others also headed to Idlib this year from suburbs of the capital Damascus and the central city of Homs as part of population transfer deals with the government.
Regime forces take more oil fields in Raqqa
Syrian regime forces backed by heavy Russian air strikes seized a string of oil wells in southwest Raqqa province on Saturday.
The development comes as retreating Daesh militants battle to defend their remaining territory in the country.
Regime-owned Ikhbariyah television quoted a military source as saying the army had taken control of Wahab, al Fahd, Dbaysan, al-Qseer, Abu al Qatat and Abu Qatash oil fields and several other villages in the desert area that lies in the southwest of Raqqa province.
The seized oil fields lie south of the town of Rasafa and its oil wells, which the regime forces took last month from the militants in their first major territorial gains inside the province.
The army and Iranian-backed militias have in the last few months been advancing east of Aleppo city and seizing swathes of territory west of the Euphrates River that militants have pulled out of to defend their de facto capital of Raqqa.
The militants are currently battling the US-backed troops in Raqqa.
The latest gains tighten the regime forces' grip on a bulge of territory stretching from eastern Hama province to eastern Homs and the edge of Raqqa and Deir Zor provinces, the Syrian Observatory for Human Rights war monitor said.
The monitor said the Russian air force intensified its strikes on several targets and towns held by the militants in the area including Uqairbat.
The Syrian regime's next goal is to retake the town of Sukhna, a gateway to the eastern province of Deir Zor that borders Iraq and likely to be the militants' last major bastion in Syria if Raqqa falls.
Heavy fighting has continued in the last 48 hours near Hail and the nearby Arak gas field that the regime forces took last month.