Next week's signing of a deal to install an American radar base in the Czech Republic as part of Washington's planned European missile defence shield has been postponed, officials said Monday.

The Czech foreign ministry said there was no threat to the deal and that the signing had been delayed because US Secretary of State Condoleezza Rice was not available for the original dates of May 5 or May 6.

The delay was "a matter of logistics," explained Czech foreign ministry spokeswoman Zuzana Opletalova.

The US plans, which are strongly opposed by Russia, will see a radar base near Prague aligned with a missile base in neighbouring Poland aimed at shooting down enemy projectiles.

Washington confirmed the delay was just due to a matter of timing, and said Rice still planned to travel to the Czech capital.

"She will be in Prague at some point," said State Department spokesman Sean McCormack. "She will be there to sign an agreement, the substance of it is done as far as I know."

According to her official program, Rice will be travelling from May 1 to 5 to Britain, Israel and the West Bank to discuss the Iranian nuclear crisis, Kosov and Middle East peace efforts.

News of the delay came as Greenpeace activists broke into the Czech military base south of the capital where the radar base is due to be situated.

"We don't want to be targets," said a banner unfurled on a hillside within the base, some sixty kilometres (40 miles) southwest of Prague.

Opinion polls show around two thirds of Czechs are opposed to hosting the US radar.

"Our protestors have established a base camp and they have also climbed trees and installed platforms," said Greenpeace spokeswoman Lenka Borakova. "We intend to say here as long as possible, until they can dislodge us." There have been a number of protests in the past few days in the Czech Republic in the build-up to the signing of deal by Prime Minister Mirek Topolanek.

Opletavola said the delay was due to the fact that Rice was "unable to coordinate" a May 5 signing date.

"There is no need to go looking for a problem," she insisted, although she refused to give any indication of a likely future date.

Preliminary negotiations on the US-Czech deal broke up last week without agreement on concerns over details of how the US would be deployed on Czech soil.

The missile defence shield is expected to become operational between 2011 and 2013.

Washington has suggested allowing Russian observers to visit the Czech site in an attempt to reassure Russia and ensure transparency, but the proposal has not progressed very far, according to various sources in Prague.

While Washington says the missile defence shield would guard against Iran, Moscow argues it would weaken Russia's nuclear defence potential.