KIROVSKE, Ukraine (AFP) - US President Barack Obama has told Russia's leader of his "deep concerns" about Moscow's increased support for separatists in Ukraine as international experts finally gathered more remains at the crash site of downed flight MH17.

In a telephone call with Russian President Vladimir Putin on Friday Obama also expressed "his preference for a diplomatic solution to the crisis", the White House said.

"I think that we have done everything that we can to support the Ukrainian government and to deter Russia from moving further into Ukraine," Obama told a unscheduled news conference in Washington.

"But short of going to war, there are going to be some constraints in terms of what we can do if President Putin and Russia are ignoring what should be their long-term interests," he admitted.

Separately, the Kremlin said the two leaders had agreed that the current standoff in Ukraine – where pro-Russian rebels are battling government forces – was "not in the interest of either country".

Moscow said Putin and Obama had agreed on the urgent need for an "immediate and stable halt to fighting in southeast Ukraine and the start of a political process".

The tragedy of the Malaysia Airlines plane, which was downed two weeks ago killing all 298 people on board, has again focused world attention on the conflict in Ukraine.

Seventy police investigators – by far the largest number to reach the location so far – finally managed to comb the scattered wreckage in the fields in eastern Ukraine on Friday.

More than 220 coffins have been sent back to the Netherlands, which lost 193 citizens in the July 17 crash. The United States says the pro-Russian rebels likely shot down the plane with a missile supplied from Russia, but Moscow and the insurgents contend the aircraft could have been brought down by a Ukrainian fighter jet.

'The biggest day'

"We are happy that we can make sure that these remains can now be sent," said Pieter-Jaap Aalbersberg, the Dutch police official sent to Ukraine to head up the mission there.

The observer mission for the Organization for Security and Cooperation in Europe (OSCE) called Friday "the biggest day" yet at the crash site as experts were able to recover several remains and passengers' belongings.

Despite the international team managing to begin work at the site the fighting that had impeded their probe continued to rage across eastern Ukraine.

The Ukrainian military said an ambush by insurgents in Shakhtarsk, a town 25 kilometers (15 miles) from the main impact site, left 14 people dead, including at least 10 soldiers.

Elsewhere around the region though, government forces relaunched their offensive to oust the separatists, after a "day of quiet" brought a brief pause to more than three months of fighting that has cost the lives of more than 1,100 people on the ground.

The Ukrainian military says it is getting close to cutting off the main rebel stronghold of Donetsk from the Russian border and the second insurgent bastion of Lugansk.

In Lugansk, officials said five civilians were killed and nine injured in clashes over the past 24 hours.

Early elections pledge

Meanwhile Ukrainian President Petro Poroshenko pledged that the strife-torn nation would have a new parliament in place in the next few months, confirming that he intends to call long-awaited legislative polls.

"In autumn there will be a new parliament that will start on reforms," Poroshenko said in a televised interview.

The fresh fighting on the ground entrenched a crisis that has pushed East-West tensions to their highest point since the Cold War. The EU and US have hit Russia with the most punitive measures since the collapse of the Communist bloc over its backing for the rebels.

Putin made his first comments on the latest tougher sanctions imposed this week by the United States and European Union against Russia in the phone call with Obama. Putin characterized them as "counterproductive, causing serious damage to bilateral cooperation and international stability overall," the Kremlin said.

Obama however insisted that "right now what we've done is impose sufficient costs on Russia that, objectively speaking, they should -- President Putin should want to resolve this diplomatically, to get these sanctions lifted, get their economy growing again, and have good relations with Ukraine."

"But sometimes people don't always act rationally," he added.

US Vice President Joe Biden, meanwhile, called Poroshenko to announce the US was giving Ukraine $8 million in new aid for the nation's border guards.

Pentagon spokesman Rear Admiral John Kirby said Russia was continuing to reinforce its military presence along the border with Ukraine.