Republican presidential hopeful Mitt Romney threw down the gauntlet to Russian leader Vladimir Putin on Monday, saying that if elected president he would show "no flexibility" on missile defense.

"I will implement effective missile defenses to protect against threats. And on this, there will be no flexibility with Vladimir Putin," Romney said in a major foreign policy speech in Virginia four weeks out from election day.

Romney was alluding to US President Barack Obama's remark to Russian President Dmitry Medvedev, caught on an open microphone earlier this year, that he would have greater flexibility to negotiate on missile defense should he win re-election in November.

Plans for a NATO-backed missile defense shield in Europe have angered Moscow, which wants guarantees that the system will not be aimed at or used against it at any time.

NATO has said the system does not target Russia but rather a threat from the Middle East, in particular Iran.

Russia in May staged the first successful test-launch of a new intercontinental missile designed to pierce NATO's new system.

The highly-symbolic launch came just four days after the alliance formally activated the first stage of the defense shield whose deployment Russia has bitterly opposed out of fears that it may target its own vast nuclear arsenal.

Russia already has hundreds of long-range missiles capable of raining down nearly 2,000 nuclear warheads on the United States.

But much of the force is built on technology developed in the Soviet era that Russia fears may become obsolete by the time NATO's shield becomes fully operational in 2018.

Putin unveiled a massive new armaments program during his successful election to a third term and made a visit to a military factory one of his first high-profile stops after his May 7 swearing in.

Putin's predecessor and protege Medvedev warned the West last year that Russia will have to deploy new rockets on the borders of NATO's European partners such as Poland should its concerns not be addressed.

It has since deployed a next-generation anti-missile radar near the Polish border and begun testing a similar station at the heart of its nuclear arsenal base in the Siberian city of Irkutsk.

The army's top general Nikolai Makarov ratcheted up the rhetoric further in May by warning that Russia reserved the right to preemptively strike NATO targets once it felt its shield posed a significant threat.