North Korea is continuing preparations to test-fire a missile but a launch does not appear imminent, Seoul's unification minister said Wednesday as a new U.S. envoy warned against such a test.

Pyongyang stepped up its provocative rhetoric of recent weeks, warning of a "merciless" retaliation against any invasion — though both Washington and South Korea dismiss talk of invading as ludicrous.

Tensions are high on the Korean peninsula, with the North announcing last week it will send a communications satellite into orbit as part of its space program — a claim neighboring governments believe is a cover for a test of a long-range missile capable of reaching Alaska.

President Barack Obama dispatched his new envoy for North Korea, Stephen W. Bosworth, to Asia for talks with China, Japan, South Korea and Russia on North Korea. The five nations are seeking to convince the North to abide by a disarmament-for-aid pact the impoverished regime signed in 2007.

With those talks stalled, analysts say North Korea is trying to grab Obama's attention and could carry out the launch toward the end of March or early next month.

Bosworth said in Beijing that the Obama administration remains committed to the resumption of the disarmament talks and the denuclearization of the Korean peninsula.

He also said the U.S. and China are united in opposing North Korea's alleged missile launch plan.

"We both believe it would not be a good idea to have a missile launch," said Bosworth, who is to travel to Tokyo and Seoul later this week.

Earlier Wednesday, South Korea's Unification Minister Hyun In-taek told reporters that he believes "various preparations are progressing" in North Korea for its missile test, though "a launch does not appear to be very imminent." He did not elaborate.

The appointment of hard-liner Hyun to the ministry in charge of relations with Pyongyang has further soured inter-Korean ties already at their lowest point in years over President Lee Myung-bak's tough stance on the North. Upcoming joint military exercises between South Korean and U.S. troops also have touched off North Korean nerves.

The U.S. military has some 28,500 troops in South Korea to help monitor a 1953 cease-fire brokered by the United Nations. Joint exercises are due to begin next week.

Pyongyang accused the U.S. of plotting to attack the North. "If the enemy intrudes ... even 0.001 mm" into the North, the regime will deal the enemy "a thousand-fold retaliatory blow," the communist state's main Rodong Sinmun newspaper said Wednesday.

Washington and Seoul have said the exercises are a routine annual drill.

Generals from the North summoned U.S. military officers representing the U.N. Command to the Demilitarized Zone Monday — their first talks in nearly seven years — but the talks ended without progress. South Korean media said the North demanded the allies abandon the exercises during the talks, but the U.S. said the drill will go ahead as planned.

Senior military officials from North Korea and the command will meet again Friday, a South Korean Defense Ministry official said on condition of anonymity, citing department policy.

In Australia, Lee urged Pyongyang not to launch a missile, the Yonhap news agency reported.

"North Korea has taken such actions as firing a missile in the past at times that it sees as appropriate," Lee said. "I believe it is again trying to take such a strong action because a new U.S. administration has been inaugurated."

Washington, Seoul and Tokyo have said that a launch, whether a missile or a satellite, would violate a U.N. Security Council resolution banning the North from any ballistic missile activity.

North Korea unsuccessfully test-fired a long-range missile in 2006 but is believed to have made improvements in its missile capabilities since then.