BRUSSELS, Belgium — The top U.S. missile defense official warned Thursday that the ballistic threat from Iran and North Korea was rapidly growing and defended Washington's plans to base parts of its anti-missile shield program in Central Europe.

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But NATO called for more evaluation of the potential danger, saying there was "no assessment of the immediacy of that threat to NATO and Europe."

Washington is in talks with Poland on the deployment of 10 interceptor missiles in the country, and with the Czech Republic on a radar base. The U.S. says the two sites are need to supplement bases in Alaska and California and create a network to defend Europe and North America from a potential attack by Iran or North Korea.

"The missile defense system addresses a real and growing threat. We believe it's prudent to stay ahead of this threat," Patricia Sanders, executive director of the U.S. Missile Defense Agency, said at a hearing at the European Parliament.

She said the planned system was purely defensive, "delivering a package to a point in space where it can destroy a missile."

She did not give details on which country would get ultimate responsibility for firing the interceptors, but said control had to be "centrally located."

Moscow has denounced the plan as a threat to Russia's nuclear deterrent and later came up with a counterproposal for a radar site in Azerbaijan.

Some EU leaders have expressed concern over the go-it-alone attitude of Poland and the Czech Republic. EU foreign policy chief Javier Solana said placing components of the shield of EU soil might affect the 27-nation bloc's relations with third countries.

NATO officials have suggested that talks focus on combining NATO's short-range system with the separate U.S. strategic shield to provide missile defense for all 26 allies.

NATO has warned Iran has tested missiles with a 1,243-mile range, which could hit Turkey, Bulgaria, Romania or Greece. Reports said North Korea has shipped missiles with a 2,175-mile range, which could target countries as far as Germany.

But a NATO official cautioned that more evaluation was needed.

"While there is agreement among NATO nations that the ballistic missile threat exists and is increasing, it is fair to say there is currently no assessment of the immediacy of that threat to NATO and Europe," Peter Flory, NATO Assistant Secretary General for Investment, told the hearing.

"The alliance has not settled on a consensus on the role of missile defense in addressing this threat," he said.

Poland has said it could conclude an agreement to host U.S. intercepters by September. The Czech Republic would need at least six months to discuss the proposal to set up radar base, said Jana Hybaskova, a Czech member of the European Parliament.