Syrian regime war planes on Tuesday bombed rebels spearheading an assault against two military bases, as Russia praised Damascus for "very actively" cooperating with chemical weapons inspectors.
The fighting underlined comments from UN Secretary General Ban Ki-moon, who on Monday warned that inspectors overseeing the destruction of Syria's chemical arsenal face unprecedented danger.
In a report to the UN Security Council seen by AFP, Ban said it would take 100 foreign experts to complete "an operation the likes of which, quite simply, has never been tried before".
The fighting in the northwestern province of Idlib began on Monday, when rebels launched an operation dubbed "The Earthquake" against the Wadi Deif and Hamidiyeh bases.
Around 25 rebel brigades joined forces for the assault, which sparked fierce fighting around the nearby opposition-held town of Maaret al-Numan, a watchdog said.
The Syrian Observatory for Human Rights said 10 regime troops and five rebels had been killed in the clashes.
The monitoring group reported regime air strikes on areas of Maaret al-Numan as well as on the nearby areas of Maarshamsha and Deir al-Sharqi, which it said caused casualties.
Russian President Vladimir Putin meanwhile praised Syria's President Bashar al-Assad for his government's cooperation with the team of international weapons inspectors.
"Doubts about whether the Syrian leadership would adequately respond to the decision taken on chemical weapons -- these doubts did not bear out," Russian news agencies quoted Putin as telling reporters at a regional summit in Indonesia.
"The Syrian leadership has very actively joined this work and is acting in a transparent manner, helping international agencies," said Putin.
A team of experts from the United Nations and the Hague-based Organisation for the Prohibition of Chemical Weapons arrived in Damascus a week ago to begin verifying the details of Syria's chemical arsenal and destroying and disabling it.
They were dispatched under the terms of a United Nations Security Council resolution passed on September 27, which enshrines an US-Russian deal for Damascus to relinquish its chemical weapons.
The deal averted threatened US military action in response to an August 21 sarin attack outside Damascus in which hundreds of people died.
Ban: 'Dangerous' work for inspectors
Ban on Monday warned of the dangers the team faces in Syria, where more than 115,000 people have been killed since March 2011, according to the Observatory.
The UN chief highlighted the threat to the experts and Syrian civilians from the sarin, mustard gas and other chemical weapons which are to be moved amid the "carnage" of Assad's daily battle with rebels.
The experts will have to work in "dangerous and volatile" conditions, particularly in urban areas such as Damascus, Homs and Aleppo, Ban warned.
"Heavy artillery, air strikes, mortar barrages and the indiscriminate shelling of civilians areas are commonplace and battle lines shift quickly," he added.
Russia and the United States have the ambitious aim to seize and destroy Syria's huge chemical weapons arsenal, estimated at 1,000 tonnes, by mid-2014.
Some 19 OPCW arms experts and 16 UN logistics and security personnel are in Syria and have started to destroy weapons production facilities.
Ban recommended the team be increased to about 100 scientists, logistics and security experts who will stay for up to a year.
The mission will have bases in Damascus and Cyprus.
Syria has already made a declaration of its weapons facilities. And the UN resolution set a November 1 deadline for the eradication of production and chemical mixing facilities.
That work started Sunday when the first "missile warheads, aerial bombs and mixing and filling equipment" were cut up and destroyed, according to the UN.
The Security Council resolution passed on September 27 threatened "measures" under Chapter VII of the UN Charter if an infringement of the disarmament accord is proved.
Chapter VII allows for possible military action or sanctions.
US Secretary of State John Kerry has welcomed the progress made so far.
"I think it's extremely significant that yesterday, Sunday, within a week of the resolution being passed, some chemical weapons were being destroyed," he told reporters in Indonesia on Monday.
"I think it's a credit to the Assad regime, frankly. It's a good beginning and we welcome a good beginning."