Russia moved tactical nuclear warheads to within miles of its borders with NATO countries as recently as late spring, complicating US ties with Moscow, a report said Tuesday, as Congress mulled a new arms control treaty.

The Wall Street Journal cited US officials who described the movements and cautioned that they ran afoul of pledges Moscow made as early as 1991 to remove the weapons from outposts near Eastern European NATO allies such as Poland.

US diplomatic cables out of Moscow leaked just before the Journal report highlighted a flurry of negotiations between the two former Cold War adversaries over their nuclear stockpiles, tactical weapons and a controversial US-NATO missile defense system in countries that border Russia.

US Senator John McCain, the top Republican on the powerful Senate Armed Services Committee, said the report was cause for concern.

"It's very disturbing because it's a violation of a commitment they made back in 1991, and they did not inform us," McCain told AFP.

"It argues for a strict verification on any treaty, any agreement on which we're engaged."

The Russian deployment of the weapons -- which unnamed officials said has been expanded several times in recent years -- was being seen as an apparent hedge against the US-NATO missile defense system.

On Tuesday Russian President Dmitry Medvedev warned that failure by Russia and the West to agree on a new missile shield for Europe could spark "a new round of the arms race" that would see Moscow deploy new weapons systems.

Former US president George W. Bush agreed on a plan for the US anti-nuclear missile shield, in part to ward off any ballistic threat from Iran, that envisaged installation of interceptors and long-range missiles in Poland and a radar system in the Czech Republic.

But after Russia objected that its security was threatened, President Barack Obama modified the plans last year, opting for medium- and short-range missile interceptors in Poland.

US diplomatic cables divulged recently by WikiLeaks suggested tensions last year over the missile defense shield.

In an April 2009 cable, a US official wrote that Russia warned it "would put missiles in Kaliningrad," a Russian enclave between Poland and Lithuania, as a countermeasures against the US-NATO shield.

A top Russian commander on Tuesday dismissed the Journal report, which mentioned a Kaliningrad deployment.

"We never deployed any nuclear missiles in the Kaliningrad region," Chief of the General Staff commander Nikolai Makarov said, the RIA Novosti news agency reported.

Officials told the Journal that warming ties between Obama and Medvedev had fostered cooperation in vital areas, but regional suspicions remained high.

Some foreign officials have stressed that the movements could pose a graver danger than larger, strategic nuclear weapons that come under the new Strategic Arms Reduction Treaty, or New START, according to the Journal report.

Lithuanian Foreign Minister Audronius Azubalis raised his concerns directly with US Secretary of State Hillary Clinton, the paper said.

"Being a NATO member, of course, someone could say, 'Don't worry.' But when you're living in the neighborhood, you should always be more cautious," the Journal quoted Azubalis as saying.

He added that US officials "expressed worry but they also don't know too much" about the location of the tactical weapons or how they are kept.

US officials said they believe the most recent Russian deployment occurred in late spring, around the time a US Patriot missile battery was installed in Poland in May, according to the report.

One unnamed US official told the Journal that US and Russian negotiators sought to address tactical weapons, but only after the new treaty goes into effect.

"If we don't ratify START, we're not going to be able to negotiate on tactical nuclear weapons," the official said.

US Republicans, emboldened by their recent victory in legislative elections, have voiced opposition to Senate ratification of New START, which in addition to allowing for US verification of Russian compliance would reduce the nuclear arsenals of both countries.

Republicans have signaled they will block a vote on the landmark treaty this year, handing Obama a defeat on a foreign policy priority he described as "absolutely essential to our national security."

On Tuesday McCain stressed that he hoped the Obama administration could could "resolve" conservative concerns about the treaty so it could be approved by the Senate by year end.