Image copyright Reuters Image caption Armed pro-Russia groups remain in control of seized buildings in eastern Ukraine

The US says Russian military aircraft have entered Ukrainian airspace on several occasions, amid rising tension in the east of the country.

A Pentagon spokesman called on Russia to "de-escalate the situation".

The statement came as the G7 group of industrialised countries agreed on Saturday to "swiftly" impose fresh sanctions on Russia over Ukraine.

Meanwhile, talks are under way to secure the release of international observers seized by separatists.

Russia has tens of thousands of troops deployed along its side of the border with Ukraine as pro-Moscow separatists continue to occupy official buildings in a dozen eastern towns, defying the government in Kiev.

'Dangerously destabilising'

Russia has accused the West of wanting to "seize" Ukraine.

In a statement on Friday, Pentagon spokesman Col Steven Warren said Russian aircraft had entered Ukrainian airspace several times in the past 24 hours.

He gave no further details, but called on Moscow to take "immediate steps to de-escalate the situation".

US Secretary of Defense Chuck Hagel earlier described Russian activity along the Ukrainian border as "dangerously destabilising" and "very provocative".

Image copyright DigitalGlobe via Reuters Image caption Images released by Nato this month appeared to show a Russian military build-up near the Ukrainian border

Image copyright AFP Image caption Ukrainian troops have set up checkpoints around Sloviansk where buildings are occupied

Last week, Ukraine and Russia struck an agreement in Geneva calling for separatists to leave official premises and give up their arms. The pact included an amnesty for those who left peacefully.

But so far pro-Russian activists have refused to give in to the demands.

A joint statement from the G7 said Russia had taken "no concrete actions in support of the Geneva accord".

"We have now agreed that we will move swiftly to impose additional sanctions on Russia," the statement said.

"Given the urgency of securing the opportunity for a successful and peaceful democratic vote next month in Ukraine's presidential elections, we have committed to act urgently to intensify targeted sanctions and measures to increase the costs of Russia's actions."

White House sources told reporters that sanctions could be in place as early as Monday.

It is unclear what form they will take. Under current US and EU measures, assets freezes and travel bans have targeted a number of Russian officials.

On Friday, Ukraine's interior ministry said armed separatists had seized seven representatives from the Organisation for Security and Co-operation in Europe (OSCE), as well as five Ukrainian army personnel and a bus driver.

Pro-Russian leaders in Sloviansk confirmed the bus had been stopped near the town of Sloviansk and said they were checking the identities of those on board.

The self-proclaimed mayor of Sloviansk, Vyacheslav Ponomaryov, said at least one passenger had been carrying maps showing separatist checkpoints in the area, which suggested "their involvement in espionage".

A 'bloody crime'

Last weekend, Mr Ponomaryov broadcast an appeal to President Putin asking for Russian troops to protect the city from "fascists" after three of his men died in a gunfight.

Tensions have risen in recent days as Ukraine launched military raids to try to regain occupied buildings.

At least two separatists have been killed in the raids which Russian Foreign Minister Sergei Lavrov described as a "bloody crime".

Mr Lavrov repeated the accusation made previously by Moscow that Ukraine was waging a war against its own people.

Unrest in Ukraine began last November over whether the country should look towards Moscow or the West.

Last month, Russia annexed Ukraine's mainly ethnic-Russian Crimean peninsula. This followed a referendum in the region that backed joining the Russian Federation but which the West and Kiev deemed illegal.