Just hours after US Secretary of State Condoleezza Rice called on North Korea to make a "strategic" choice to drop its nuclear weapons, Pyongyang said it had already taken measures to boost its arsenal.

"In the face of the enemy's mounting war provocations, our armed forces and the people have already been fully prepared for war mobilizations in order to bust any aggression attempts at one sweep and taken the decisive measure of increasing our nuclear arsenal," North Korea said in a radio broadcast monitored here by Yonhap news agency.

Rice urged Pyongyang to return to six-party talks to resolve the standoff during her six-nation Asian tour which ended Monday in Beijing. She visited Japan, South Korea and China, key players in the standoff.

With the talks in limbo, Rice pushed hard for a new round, indicating Washington's patience may be running out and other options were being considered.

"It is true that we need to resolve this issue. It cannot go on for ever," she said in Seoul on Sunday.

In Beijing, she warned North Korea that Washington was considering "other options" to talks. Experts noted that hawks have been pushing for referral of the case to the United Nations.

Backing up Rice's call for North Korea to return to dialogue, the White House warned diplomatic efforts cannot "drag on forever."

"Secretary Rice was saying what we've said, the time to come back to the talks is now. She expressed that this could not drag on forever; we need to resolve this issue," spokesman Scott McClellan said.

While Rice applied verbal pressure, she also appeared conciliatory, saying that through talks, North Korea would receive what it was asking for in terms of the "respect that they have desired and ... the assistance that they need."

North Korea has demanded an end to US "hostility" and rewards for dismantling its nuclear weapons drive. It has also requested direct talks with the United States to end the standoff.

Rice repeated throughout her trip that Washington had no intention of attacking North Korea and indicated its energy needs could be met through six-party talks where direct dialogue -- though not separate negotiations -- could also take place with Washington.

She also referred to North Korea as a sovereign nation rather than an "outpost of tyranny," a term she used in January that inflamed the North Korean leadership, while refusing Pyongyang's demand for an apology.

North Korea attended three rounds of inconclusive six-party talks along with South Korea, China, Japan, Russia and the United States but failed to show up for a fourth round in Beijing last September.

In February, Pyongyang announced it possessed nuclear weapons and was withdrawing from the negotiations indefinitely.

Several days later, however, North Korean leader Kim Jong-Il indicated he could return to talks if the conditions were right.

Coinciding with Rice's visit to Seoul on Saturday, South Korea and the United States launched week-long military drills dubbed preparations for an invasion by North Korea.

Washington believes North Korea possesses one or two crude bombs and may have reprocessed enough plutonium for half-a-dozen more, from spent fuel rods at its Yongbyon nuclear complex.

The nuclear standoff erupted in October 2002 when the United States accused North Korea of operating a program based on highly enriched uranium.

Pyongyang denied that charge but restarted a plutonium-based program frozen under a 1994 arms control agreement.