US Secretary of State John Kerry heads to Moscow this week for talks with President Vladimir Putin, hoping to build momentum for peace in Syria after a partial Russian withdrawal and to restore a fragmenting ceasefire in Ukraine.

But few experts expect Washington's top diplomat to make much headway with a Kremlin that has achieved its short-term goals and is seeking new victories.

Having ensured that he has a seat at the top table of world diplomacy and that his allies in Damascus are in no immediate danger of defeat, Putin has ordered the bulk of his forces out of Syria without suffering great losses.

Now, observers say, his separatist proxies in Ukraine are increasing pressure on the ceasefire line there, hoping that Europe's commitment to renew sanctions will waver this summer before Russia's September parliamentary polls.

Joerg Forbrig, a fellow at the German Marshall Fund of the United States, compared this week's trip by Kerry to Moscow to one he made last year to see Putin in Sochi after Russia helped Washington to negotiate the Iran nuclear deal.

Russia carried through on its support for the Iran deal, helping ship out Tehran's uranium stockpile, and now Kerry wants Moscow to help push through a Syrian peace plan. 

‘So he goes to Moscow to see if this positive momentum can be cultivated and perhaps extended. I don't think it can,’ Forbrig, an expert on central and eastern Europe, told AFP.

‘Russia has basically got out of this intervention everything that it needed,’ he argued, suggesting Moscow will be content to see peace talks drag on indefinitely if its interests are not again threatened.

‘It has a place at the negotiating table, it is sure to be part of the political process that is now underway. It has sold to its own public a very successful intervention that installed a ceasefire in this five-year-old conflict. So I think they've cashed in now.’

- New incentive? -

Steven Pifer, a former ambassador to Ukraine and director for Russia on the US National Security Council, is less sure that, having brought Bashar al-Assad's Syrian regime to the negotiating table, the Kremlin will scale back its support for the UN-mediated peace talks in Geneva.

‘After the last week, with Putin having in a way declared victory, the Russians now have an incentive to see the negotiations succeed,’ Pifer -- now a senior fellow at the Brookings Institution, a Washington think tank -- told AFP.

‘After the Russian intervention, Assad's certainly in a more stable place than he was last summer, but if it deteriorates again, it looks bad for Putin if he has to send the military back in. So that may bring more into line the American and Russian aims.’

Fyodor Lukyanov, editor-in-chief of the journal ‘Russia in Global Affairs’, told AFP that this odd couple diplomatic partnership is the only hope for peace in Syria.

‘Apart from Russia and the United States, there are no other motors behind the process, a little like in the good old days of the great powers,’ he said, reflecting Moscow's nostalgia for its Cold War eminence in world diplomacy.

US and Russian spokesmen have confirmed Syria will be a key issue at the Kremlin talks on Thursday, but the crisis in Ukraine will also be on the agenda, and here too Putin may sense an opening to score points against the West.  

In a sign of the allies' close coordination, the US secretary of state will fly into Moscow on Wednesday hot on the heels of another key player, German Foreign Minister Frank-Walter Steinmeier, who is also due at the Kremlin.

Germany and France are leading the western push for the implementation of the 2014 Minsk Protocol, under which Moscow is to calm its separatist allies in Ukraine while Kiev reforms its constitution to hold new elections and grant the Donbass region federal autonomy.

- Ceasefire violations -

Last week, a senior State Department official told reporters that recent weeks have seen a stark increase in ceasefire violations, a development he attributed to Putin's desire to turn up the political heat on Kiev.

US policymakers are sympathetic to Kiev's dilemma, arguing that Russian provocation acts as a ‘violent veto’ on its attempts to pass reform -- but some in Europe are becoming frustrated with Ukraine's failure to prepare for elections.

‘If there's an uptick in violence, then Berlin gets very nervous, because they have no Plan B,’ Forbrig said.

‘They have absolutely no alternative to Minsk and they will ... increase their pressure on Kiev because that's the only point where they have leverage.’

‘This undermines the unity on sanctions that we've seen so far,’ he said, noting that some of Germany's partners in Europe are already lukewarm on maintaining an embargo against Russia and may look for an excuse to back out.

So is Kerry wasting his time? Or worse, is he handing Putin a propaganda coup by heading once again to pay court to the Kremlin?

‘It's his job. He's a top diplomat. Their job is to go, try to talk, try to negotiate. That may seem naive to us, but they have to try,’ Forbrig said.

‘But I don't see this resulting in anything to be honest.’