Washington (AFP) - US President Barack Obama called his Russian counterpart Vladimir Putin Friday to urge him to resolve the Ukraine crisis diplomatically, but acknowledged that sometimes people don't "act rationally."
"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 surprise news conference.
"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."
The White House said in a statement that during the call the US leader had repeated his concern about Moscow's alleged breach of the 1987 Intermediate-Range Nuclear Forces Treaty, under which the United States and Russia agreed not to develop medium-range cruise missiles.
"The president reinforced his preference for a diplomatic solution to the crisis in Ukraine, and the two leaders agreed to keep open their channels of communication," a White House statement said.
Dismissing what he called "skepticism" about Washington's ability to coordinate with its European allies, Obama pointed to the new round of European Union and US sanctions imposed on Moscow this week for fomenting the unrest in eastern Ukraine, and stressed "we can't control how Mr. Putin thinks."
"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," Obama told reporters.
"But sometimes people don't always act rationally and they don't always act based on their medium- or long-term interests. That can't deter us, though. We've just got to stay at it."
Separately, the Kremlin said the two leaders had agreed that the current situation in Ukraine -- where pro-Russian separatists are battling the government -- was not in either country's interest.
Putin also, according to the Kremlin, told Obama that Western economic sanctions were "counterproductive, causing serious damage to bilateral cooperation and international stability overall."
Vice President Joe Biden, meanwhile, called Ukrainian President Petro Poroshenko to announce the US was giving Ukraine $8 million in new aid for the nation's border guards, amid what they called "Russia's deeply destabilizing efforts to continue supplying weapons to its proxies in eastern Ukraine."
The funds would be used for "engineering equipment for improving infrastructure along Ukraine's borders, transport and patrol vehicles," as well as surveillance equipment and small boats for maritime patrols.
Pentagon spokesman Rear Admiral John Kirby said Russia was continuing to reinforce its military presence along the border with Ukraine.
"It continues to be north of 10,000, the numbers, but it fluctuates," Kirby told reporters, adding troops were "close to the border, within 50 kilometers of the border -- closer than what we saw back in the spring."