Ukraine's government says it fears a concentration of Russian troops on the countries' border could be the prelude to an invasion

Deputy Foreign Minister Danylo Lubkivsky issued the warning Friday at the United Nations, saying that his country faces a possible Russian invasion "at any moment" and vowing that Ukraine would defend itself.

Concerns about Russia's intentions, meanwhile, prompted interim Prime Minister Arseniy Yatsenyuk on Saturday to shorten his visit to Rome, where he was received by Pope Francis.

Russian military activity on its side of the border was slightly less intense on Saturday, Ukrainian Defense Minister Mikhail Koval acknowledged, although he said Russian troops had moved to within 2-3 kilometers (1.2-1.8 miles) of the frontier.

"Four IL-76 transport planes flew along our border at midday, but did not cross at any moment" to the Ukrainian side, Koval said at a press conference.

He said the troop movement may also be aimed at intimidating the Ukrainian government into halt its "anti-terrorist operation" against pro-Russian separatists in the country's southeast.

"I should say that the anti-terrorist operation is continuing, albeit not at a rapid pace," Koval added.

Ukrainian forces did not undertake actions Saturday against separatist militants entrenched in Slovyansk, a stronghold of a pro-Russian uprising against the government in Kiev.

Militants in that city seized seven Western military observers (three Germans, a Pole, a Dane, a Swede and a Czech), five Ukrainian officials and the driver of the bus in which they were riding on Friday.

The militants say the group from the Organization for Security and Co-operation in Europe was "detained" as suspected NATO spies.

Ukrainian acting President Olexandr Turchynov, for his part, did not hesitate to accuse Russia of being behind the "kidnapping" of the Western military observers.

"This crime could not have been committed without the direct authorization or order of the Russian government, which coordinates and supports terrorists who occupy buildings, take hostages, torture and kill people," Turchynov was quoted as saying by his press office.

Russia's Foreign Ministry said Saturday in Moscow that it was taking measures to "resolve the situation" of the detained Western military observers.

It accused the Ukrainian authorities of responsibility for the detentions, saying they should have previously "arranged issues related to the permanence, activity and safety of the observers in regions where they do not control the situation and where they have launched a military operation against the inhabitants of their own country."

The rising tensions on the Russian-Ukrainian border prompted the G7 group of developed nations to impose new sanctions on Russia for allegedly violating a peace deal reached earlier this month in Geneva by continuing to support pro-Russian militants in eastern Ukraine.

The European Union also will hold a meeting Monday to weigh the possibility of new sanctions on Russia.

Moscow, for its part, denies provoking unrest in eastern Ukraine but has warned its neighbor not to use force against separatist militants in that region.

Long-simmering tensions between pro-European western Ukraine and the country's eastern region, which has close ties with Russia, were exacerbated by the ouster in late February of President Viktor Yanukovych, a Kremlin ally.

In the wake of his removal from office, Moscow sent troops to the strategic region of Crimea.

It subsequently annexed that peninsula last month - a move the West considers illegitimate - after its mostly Russian-speaking population voted in a referendum to break off from the Ukraine and rejoin Russia.

Moscow says Yanukovych was removed from office on Feb. 22 by far-right Ukrainian nationalists and that it moved to protect ethnic Russians and Russian interests in Crimea following that development.

The crisis that led to Yanukovych's ouster erupted at the end of November, when the Ukrainian president backed away from plans to ink a pact with the European Union and instead signed a $15 billion financial-aid package with Russia. EFE