(AFP) Syrian President Bashar al-Assad said Thursday he has never faced pressure from Russia to step aside, as US Secretary of State John Kerry headed to Moscow seeking to revive stalled peace efforts.

Speaking to NBC News in Damascus, Assad insisted his Russian counterpart Vladimir Putin and Foreign Minister Sergei Lavrov had never raised the issue of his departure or a political transition.

“Only the Syrian people define who’s going to be the president, when to come, and when to go. They never said a single word regarding this,” he said.

Assad’s fate is a key question in efforts to bring about a negotiated settlement to Syria’s five-year civil war.

Washington and Moscow have backed a roadmap that calls for a nationwide ceasefire and Geneva-based talks on “political transition.”

But there has been little progress towards a hoped-for resumption of talks this month, and the prospects for a political transition beginning by August, as laid out in the road map, now appear slim.

Kerry was due to arrive later Thursday in Moscow, a close ally of Assad’s government that launched air strikes in support of regime forces last September.

Kerry said before leaving Washington that he would meet Putin “to see if we can somehow advance this (the peace process) in the important ways that people want us to.”

Kerry ‘extremely frustrated’

Moscow and Washington brokered a landmark partial ceasefire in Syria in February but it has since all but collapsed amid continued heavy fighting.

Kerry’s spokesman John Kirby told reporters his boss was “extremely frustrated” with the failure of peace efforts and “his patience was growing thin.”

In Washington, many observers have criticized Kerry’s outreach to Russia on Syria, arguing he has been strung along by Putin as he seeks to protect his client Assad.

But Kirby insisted the administration is not being naive, and that Thursday’s visit to Moscow, Kerry’s third this year, would “probe the sincerity” of Putin’s promises.

In Turkey meanwhile, Prime Minister Binali Yildirim restated his country’s opposition to Assad, saying there could be no solution to the conflict while he was in office.

“As long as Assad is there, the problem won’t be solved,” he told the BBC in an interview broadcast late Wednesday.

Syria’s conflict began in 2011 with the brutal repression of anti-government demonstrations and has evolved into a complex multi-front war that has left more than 280,000 dead and forced millions from their homes.

Efforts to bring an end to the war have taken on greater urgency since the emergence of the Islamic State group (ISIS), which seized control of large parts of Syria and neighboring Iraq in mid-2014.

The jihadist group has committed widespread atrocities in areas under its control and organized or inspired a wave of attacks across the Middle East and in Western cities.

A US-led coalition is carrying out air strikes against the jihadists in Syria and Iraq and recent months have seen ISIS lose significant territory.

The Washington Post reported Thursday that Washington was to offer to cooperate with Russia in joint military action against ISIS and the Syrian branch of Al-Qaeda, Al-Nusra Front.

In Paris before heading to Moscow, Kerry did not deny the report, but refused to discuss the proposal in detail until he had been to the Kremlin.

Kremlin spokesman Dmitry Peskov declined to comment on the report before Moscow had received Washington’s proposal.

Colvin ‘responsible’ for death

According to the Post, which cited sections of what it said was a draft agreement, US and Russian commanders would set up a joint command and control center to direct intensified air strikes against the groups.

Such a deal is likely to face criticism that it amounts to a tacit acceptance of Putin’s efforts to shore up Assad’s regime.

In his interview with NBC, Assad also said a US reporter killed in alleged Syrian government bombardment in 2012 was responsible for her own death.

Marie Colvin, a 56-year-old war correspondent working for British newspaper The Sunday Times, died in the rebel-held Baba Amr district of Syria’s third city Homs.

“It’s a war and she came illegally to Syria. She worked with the terrorists, and because she came illegally, she’s been responsible of everything that befall on her,” Assad said, speaking in English.

Asked if she was responsible for her own death, Assad replied “of course,” though he denied that his forces had targeted her.

His comments came days after relatives of Colvin filed a lawsuit in a US court alleging Assad’s regime targeted her to stop her covering government atrocities.

The suit, based on information from captured government documents as well as defectors, claims the Syrian military intercepted Colvin’s communications and unleashed a barrage of rocket fire on her location, killing her and French photographer Remi Ochlik.