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, according to dpa. 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.