The U.S. and South Korea have pinpointed 11 underground sites in North Korea where a third nuclear test could be conducted, a newspaper reported Monday ahead of a summit between the two allies on the communist regime's growing atomic threat.

South Korean President Lee Myung-bak departed on a trip to Washington for summit talks Tuesday with President Barack Obama, which are expected to be dominated by the North's nuclear and missile programs.

Tension on the Korean peninsula spiked after North Korea declared Saturday it would step up its nuclear bomb-making program by producing more plutonium and uranium, two key ingredients.

The North also threatened war with any country that tries to stop its ships on the high seas as part of new U.N. Security Council sanctions passed in response to Pyongyang's May 25 nuclear test.

North Korea is believed to have enough weaponized plutonium for at least half a dozen atomic bombs, and a U.S. government official said last week that Pyongyang may be preparing for another nuclear test, its third.

The official spoke on condition of anonymity in order to discuss the unreleased information and provided no details.

U.S. and South Korean intelligence were keeping a close eye on signs of an impending test.

"South Korean and U.S. intelligence authorities have spotted 11 key underground facilities in North Korea and embarked on an intensive lookout," South Korea's mass-circulation JongAng Ilbo newspaper reported.

It quoted an unnamed government intelligence official as saying the allies have mobilized spy satellites and human intelligence networks to check for vehicle movements and other unusual activity.

The mobilization is based on "intelligence that North Korea can conduct a third nuclear test in protest against the U.N. Security Council sanctions," the paper quoted the official as saying.

Also Monday, Yonhap news agency quoted an unidentified intelligence official as saying the North may have already built two to three underground test sites near its known Punggye-ri site in the remote northeast, where it conducted its first and second tests.

South Korea's Defense Ministry and National Intelligence Service said they could not confirm the reports.

A news report from Moscow quoted an official in the Russian military general staff as saying there has been a decrease in visible activity around North Korea's nuclear facilities in recent days.

This could either indicate that the North has prepared for a new underground nuclear test or is taking a break, according to the state-owned RIA-Novosti news agency. It did not name the official, and the general staff could not immediately be reached for comment.

North Korea has also been preparing to fire an intercontinental ballistic missile capable of striking the United States. It says the nuclear and missile programs are a deterrent against the United States.

However, Washington fears that cash-strapped North Korea will sell its nuclear technology to rogue nations, spreading the atomic threat.

The regime has also warned it cannot guarantee the safety of South Korean and U.S. navy ships sailing near the disputed western sea border, raising the specter for a maritime confrontation. The area is the scene of two bloody maritime skirmishes between the Koreas in 1999 and 2002.

The strong ties between South Korea and the United States are a thorn in the side of wartime foe North Korea, which accuses the two countries of plotting an attack to topple the communist regime. The allies deny harboring any such intention.

But President Lee of South Korea said his country's ties with the United States are "key" at a time of "intensifying" security crisis because of North Korea's nuclear and missile tests.

"I will use this summit to reconfirm the strong Korea-U.S. alliance," Lee said in a radio speech before his departure.

The two Koreas remain technically at war because their three-year conflict ended in a truce, not a peace treaty, in 1953, and they remain divided by a heavily fortified border. The U.S. has 28,500 troops in South Korea.

The two Koreas signed an accord to ease military tensions and promote economic cooperation nine years ago Monday. However, ties have significantly frayed since Lee, a conservative who advocates a hard-line approach, took office last year.

On Monday, a group of South Korean conservative activists sent about 100,000 leaflets by balloons across the border into the North, to criticize its nuclear and missile programs.