The predominantly Kurdish Syrian Democratic Forces (SDF), backed by Russian air strikes, have captured a former military airbase near the border with Turkey after heavy clashes with rebel factions, a monitoring group and activists have said.

The fierce fight for control of Aleppo province continued on Thursday as diplomats from major powers sat down in Munich to discuss the Syrian peace process - including a proposal from Moscow for a nationwide ceasefire to start in March.

The Syrian government, backed by Russian air strikes, launched a major offensive from the north of Aleppo and captured several strategically important towns earlier this month.

The offensive has led to the displacement of more than 50,000 civilians from Aleppo, tens of thousands of whom have amassed in camps at the Turkish border.

On Thursday, Kurdish fighters captured a former military airbase near the rebel-held town of Azaz in Aleppo province, the UK-based Syrian Observatory for Human Rights reported.

"The Syrian Democratic Forces have captured the Menagh airbase following heavy clashes with Ahrar al-Sham and al-Nusra front," Taj Kordsh, an SDF spokesman, told Al Jazeera on Thursday.

"Russian air strikes have backed and helped the SDF fighters capture this airbase which is close to the Turkish border."

Sami Kekhia, a Syrian activist on the Turkey-Syria border, confirmed to Al Jazeera that the airbase had been captured by the Kurdish fighters.

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The SDF was founded in Syria's mainly Kurdish northeastern region in October 2015, and is made up of at least 15 armed factions - mostly fighters from the Kurdish People's Protection Units (YPG) and the Free Syrian Army.

The fighters include Christians, Arabs and about 500 foreign fighters, Kordsh said, adding that some groups in Aleppo and Idlib pledged allegiance to the SDF last month.

Ceasefire proposal

The latest clashes come as world leaders, including the U.S., Russia, Saudi Arabia and Iran, met in Munich on Thursday with the aim of restarting the peace talks between the government and Syrian opposition.

Russia has proposed a nationwide ceasefire and is waiting for a reaction from international powers, Russia's Foreign Minister Sergey Lavrov said on Thursday.

"We have put forward our proposals on a ceasefire, which are rather concrete," Lavrov said before meeting US Secretary of State John Kerry, according to the TASS news agency.

"We are waiting for an answer from the US before offering it for consideration of the International Syria Support Group."

Al Jazeera's Rory Challands, reporting from Moscow, said that the US and its allies would probably view Russia's truce offer with scepticism.

"The US concerns are that this gives the offensive that's currently ongoing in the Aleppo region more time to push the rebels back, possibly going so far as a full military victory," said Challands.

The Syrian government holds the west of Aleppo, Syria's largest city, while the rebels hold the east, but the situation is largely reversed in the countryside.

Thousands flee Aleppo

At least 50,000 Syrians have fled the fighting in Aleppo, the International Committee of the Red Cross said on Wednesday, adding that water supplies have been disrupted in some parts of the province.

Salah Eldine, an activist in Aleppo, told Al Jazeera on Thursday that hundreds of internally displaced Syrians have arrived to the town of Daret Ezzeh.

"They are sending people by bus from the town of Azaz, near the border with Turkey, to Afreen and finally to Daret Ezzeh.

"We have all opened our houses to them, but we are running out of supplies as many buses continue to arrive. In the past 24 hours at least 50 buses full of people have arrived. They are in terrible condition," Eldine said.

"The fighting around has intensified and has not stopped in days. The air strikes have not stopped either. Who knows, we might all have to leave here soon," he added.

The Observatory reported on Wednesday that at least 500 people, including 89 civilians, have been killed since the offensive began on February 1.