Prague, Warsaw  - With Prague due to sign a deal with the United States later in July over the location of a controversial radar facility in the Czech Republic, Warsaw Friday was holding out for more in relation to the planned missile defence shield.

Polish Prime Minister Donald Tusk rejected as "insufficient" the latest US offer in exchange for locating missiles in Poland.

Addressing journalists in Warsaw, he said the US should offer Poland "real security guarantees," but said he was still open to negotiations.

Poland's President Lech Kaczynski and his twin brother Jaroslaw, who leads the opposition, have both urged Tusk not to delay in reaching a deal with the US.

Tusk has held back in his dealings with Washington as he believes US missiles on Polish soil will increase the threat to Poland, which is why he has asked for "additional reassurances," he said Friday.

The US State Department said earlier in the week Washington remains committed to reaching a deal with Poland, but has not ruled out alternative countries if a deal cannot be worked out.

Lithuania has been the focus of speculation and the Baltic state has said it is open to the idea.

Since taking office in November, Tusk has increased demands on Washington for military assistance in return for the basing of the interceptors.

The countries have since begun negotiating a separate agreement for military assistance that could include improvements to Poland's air defences.

Tusk fears a possible attack from Russia, which sees the missile defence shield in its former Warsaw Pact satellites as a provocation, but Washington insists the shield is to prevent possible attacks from what it calls "rogue states," like Iran.

Tusk said he passed on his concerns to US Vice President Dick Cheney in a telephone conversation Thursday evening. He stressed that the US was still Poland's closest ally.

Meanwhile, speaking on a visit to Azerbaijan Friday, Czech Foreign Minister Karel Schwarzenberg confirmed Prague would sign the deal, saying the US missile defence shield was important for Europe, even if there was no immediate danger, RIA Novosti news agency reported.

Czech media speculated that the deal could be signed as early as next week when US Secretary of State Condoleezza Rice visits Prague, but the Czech parliament must still approve the plan and there is no clear majority in favour of it.

Most Czechs have said in opinion polls that they are against the defence shield.

Russia has suggested the Gabala radar facility in Azerbaijan as an alternative, but Schwarzenberg rejected this during his visit to the capital Baku, saying Gabala was too old. (dpa)