Two Ukrainian early-warning radar systems ceased data sharing with Russia overnight after a post-Soviet agreement between the two countries was cancelled, Ukraine's Space Agency said on Thursday. Quoting Moscow defence ministry officials, Russian media said Moscow had put into service its own facility in Armavir in the southern Krasnodar region to protect its southern flank, after the loss of data from the Ukrainian-based radars.

Russia cancelled a 1992 agreement on sharing radar information last year, saying the systems were outdated and that it would be "unthinkable" to have such installations in a country aspiring to join NATO.

"According to the agreement, last night the transmission of information was stopped. These stations are working properly and being used for the monitoring of space," a spokeswoman for the Ukrainian Space Agency said. Ukraine's President Viktor Yushchenko wants the country to join NATO, an ambition that infuriates Moscow which sees any NATO expansion on its borders as a threat.

Ukrainian officials said they could instead integrate the radar systems -- in Mukachevo at the Hungarian border to the far west of the country and in Sevastopol in the southern peninsula of Crimea -- with European Union and NATO countries.

Quoting the Russian military, who describe their system codenamed "Voronezh" as "state-of-the-art", Russian media said the deal on the use of Ukrainian-based radars had expired at midnight on Feb. 26.

Unnerved by U.S. plans to build a missile shield in eastern Europe -- complete with interceptor missiles to be deployed in Poland and a radar in the Czech Republic -- Moscow repeatedly offered Washington access to the Armavir radar and a site in Azerbaijan, to scan airspace as far as Iran, a U.S. foe.

Washington eventually turned down the offer.