SEOUL, South Korea - North Korea yesterday vowed to retaliate if punitive U.N. sanctions are imposed for its latest nuclear test, and U.S. officials said there were new signs that Pyongyang may be planning more long-range missile launches.

With tensions rising, the communist nation punctuated its barrage of rhetoric with yet another short-range missile launch - the sixth this week.

Perhaps more significant, U.S. officials said there were indications of increased activity at a site used to fire long-range missiles.

The officials spoke on condition of anonymity because methods of gathering information about North Korea are sensitive. The officials also said that an initial U.S. air sampling from near the underground test site was inconclusive.

Officials said the initial analysis did not prove that the North successfully completed an atomic reaction. At least one more test is coming.

South Korea's Yonhap news agency said the latest test launch was a surface-to-air missile designed to defend against aircraft or other missile attacks.

The nuclear test and flurry of missile launches, coupled with the rhetoric from Pyongyang that it will not honor a 1953 truce ending the fighting in the Korean War, have heightened concerns that the North may provoke a skirmish along the border or off its western coast - the site of deadly clashes in 1999 and 2002.

'A limit to our patience'

But officials said the heavily fortified border remained calm and Defense Secretary Robert M. Gates said that Washington did not see the situation as a crisis warranting any more troops to augment the 28,000 U.S. forces already in South Korea.

North Korea remained strident.

"There is a limit to our patience," its Foreign Ministry said in a statement carried on the official Korean Central News Agency. "The nuclear test conducted in our nation this time is the Earth's 2,054th nuclear test. The five permanent members of the U.N. Security Council have conducted 99.99 percent of the total nuclear tests."

North Korea said it conducted the test in self-defense. It has asserted the United States is planning a preemptive strike to oust the regime of Kim Jong Il and warned it would not accept sanctions or other punitive measures being discussed by the Security Council.

The draft of a U.N. resolution being negotiated in response to the North's second nuclear test calls on all countries to immediately enforce sanctions imposed after the North's first test in 2006.

A war scenario

They include a partial arms embargo, a ban on luxury goods, and ship searches for illegal weapons or material. The sanctions have been sporadically implemented, with many of the 192 U.N. member states ignoring them.

A list of proposals was sent Wednesday to the five permanent veto-wielding council members - the United States, Russia, China, Britain, and France - and the two countries most closely affected by the nuclear test, Japan and South Korea.

Experts say a new war would probably begin with artillery and missiles capable of hitting Seoul with little or no warning, followed by an attempt to invade the capital before the South could respond. Civilian and military destruction would be great, with many casualties, even if the North did not use nuclear weapons, although the consensus U.S. view is that the South would prevail.

A squadron of F-22 stealth fighters - the most advanced in the U.S. Air Force - was due to arrive today on the southern Japanese island of Okinawa, and yesterday's missile launch may have been the North's attempt to show it has the means to shoot them down, or at least make any incursion into its airspace risky.