US President Barack Obama has hit out at Russia for flouting the Geneva accord as international tensions over Ukraine continue and a cohort of US troops land in Poland for military drills.

Obama, who is in Japan as part of a multi-nation Asian tour, condemned the continued Moscow-backed takeover of government buildings in cities in eastern Ukraine. He also hinted at the possibility of further sanctions on Russia.

"We have seen them not abide by the spirit or the letter of the agreement in Geneva," Obama said at a news conference with Japanese Prime Minister Shinzo Abe.

The president said Moscow did not rein in "malicious and armed men" who have been taking over key government installations in the eastern cities of Ukraine, rendering the Geneva accord futile.

The Geneva agreement, signed last week by Russia, Ukraine, the US and EU, put in place a framework for easing tensions between Kiev and Moscow.

The accord aimed to end confrontation between Kiev and pro-Russian protesters and called on the protesters to leave the occupied government buildings.

However, a week after the accord, the ground situation has not changed, Obama said. He blamed the Russians for the worsening of the situation.

"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," the president said, according to AFP.

Obama also praised Ukraine for abiding by the Geneva agreement, saying the Kiev administration has taken "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."

Obama, however, ruled out military intervention in Ukraine, saying sanctions enforced on Russia have already made a financial impact on the country.

"We have already applied sanctions that have had an impact on the Russian economy ...We have continued to hold out the prospect, the possibility to resolve the issue diplomatically."

Earlier, Britain and the Netherlands said fighter jets were scrambled to intercept Russian military aircraft which approached the air space of Nato members.

British military aircraft were despatched "to determine the identity of unknown aircraft that approached the Nato Air Policing Area north of Scotland and could not be identified by other means," a Defense Ministry spokesman said, according to CNN.

Dutch officials also confirmed two Russian TU-95 bombers had entered the country's air space and that fighter jets were scrambled to escort the intruding aircraft as they returned.

With tension on Ukraine's eastern borders mounting and sabre-rattling intensifying, the United States sent in a small contingent of troops to Poland, in a sign of Washington's pledge to stand by its allies in eastern Europe.

An initial batch of 150 soldiers from the 173rd Airborne Brigade arrived in Poland and 450 more are expected to arrive within days. The troops will take part in military drills in Poland, Lithuania, Latvia and Estonia in the coming weeks.