US President Barack Obama accused Russia on Thursday of failing to respect an agreement to defuse the crisis in Ukraine, the latest salvo in the Cold War-like duel between Washington and Moscow over the future of the ex-Soviet nation.
Obama said Washington was ready to slap fresh sanctions on Moscow, a day after Russia, which has massed tens of thousands of troops on its border with Ukraine, warned that it was ready to strike if its interests were attacked.
"So far at least we have seen them not abide by the spirit or the letter of the agreement in Geneva," Obama said in Tokyo at the start of a four-nation tour of Asia.
"Instead we continue to see malicious, armed men taking over buildings, harassing folks who are disagreeing with them, destabilising the region and we haven't seen Russia step out and discouraging it.
"On the other side you have seen the government in Kiev taking very concrete steps, introducing amnesty law, offering the whole range of reforms with respect to the constitution, that are consistent with what we discussed in Geneva."
If Russia did not play its part, there would be "consequences and we will ramp up further sanctions," he warned.
Under the deal struck between Ukraine, Russia, the US and the EU in Geneva last week, militias in Ukraine were due disarm and give up control of seized state property.
But while Washington and Kiev have put the onus on pro-Kremlin militants holding buildings in the east, Moscow said the responsibility fell to pro-Western nationalists camping out in Kiev.
The Kremlin has an estimated 40,000 Russian troops poised on Ukraine's eastern border, prompting Washington on Wednesday to start deploying 600 US troops to boost NATO's defences in eastern European states bordering Ukraine.
The first unit of 150 US soldiers arrived in Poland on Wednesday, with the remainder due to land in Estonia, Lithuania and Latvia in the coming days.
The Geneva accord was aimed at easing the crisis that first flared in Ukraine in November, when its pro-Kremlin president backed out of an agreement to bring the country closer to the EU, sparking protests in the strategic nation of 46 million sandwiched between Russia and the European Union.
The protests eventually forced the president from power in late February and led to the installation of a pro-Western team in Kiev, prompting Russia to send troops into and annex Ukraine's Black Sea peninsula of Crimea and warn that it reserved the right to use troops to protect Russian speakers in eastern Ukraine.
Pro-Kremlin militants have since taken over state buildings in nearly a dozen towns across Ukraine's southeast, which has deep cultural and historical ties to Russia.
Kiev and Washington say Moscow is supporting the militants, allegations that Russia has denied.
The crisis in Ukraine has triggered the worst East-West crisis since the collapse of the Soviet Union in 1991, with the Cold War echoing in the rhetoric.
On Thursday Russian Foreign Minister Sergei Lavrov told state-controlled RT television that if Russia or its interests are attacked, "we would certainly respond".
"If our interests, our legitimate interests, the interests of Russians have been attacked directly, like they were in South Ossetia for example, I do not see any other way but to respond in accordance with international law," he said, referring to Russia's invasion of Georgia in 2008.
Lavrov said "The American are running the show" in Kiev, saying that a new offensive announced by the government to dislodge the rebels came immediately after a visit by US Vice President Joe Biden to Kiev.
US State Department spokeswoman Jen Psaki called Lavrov's claims "ludicrous", saying they were not based on "what is happening on the ground".
Ukraine's government on Wednesday announced it was launching another operation to try and dislodge the separatists from their positions in the southeast, with the interior ministry saying special forces had done so in one small eastern town, Svyatogorsk.
But AFP found no military units there -- only dumbfounded residents who said they had never been under rebel occupation.
The detention by the rebels of two journalists -- an American working for VICE News, Simon Ostrovsky, and a Ukrainian activist, Irma Krat -- in Slavyansk have done nothing to ease the mounting tensions.
The rebel leader in the town, Vyacheslav Ponomaryov, called Ostrovsky a "journalist provocateur" and promised "we will free him in due course".
The US State Department said it was "deeply concerned about the reports of a kidnapping" of Ostrovsky and called for Russia to organise his immediate release.
Slavyansk was also the source of gunfire that damaged a Ukrainian military reconnaissance plane on Tuesday, and the site where two bodies were found that Kiev's acting president Oleksandr Turchynov said had been "brutally tortured".
One of the two victims was believed to be a local politician and member of Turchynov's party, which the president used as justification to relaunch the military operations against the insurgents.
Russia's gas supplies to Ukraine and Europe have become another source of tensions.
The vice president of Russia's state-owned Gazprom, Alexander Medvedev, said at a Paris news conference Wednesday that Ukraine's gas debt, which he calculated would be $3.5 billion by the beginning of May, is "intolerable".
Putin has warned in a letter to the EU that Moscow could cut gas supplies in a month's time if Ukraine's bill was not paid in full.