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

Washington warned today that the U.S. would respond quickly to any moves that threaten America or its Asian allies.

“We will not stand idly by as North Korea builds the capability to wreak destruction on any target in Asia — or on us,” U.S. Defense Secretary Robert Gates said at a regional defense meeting in Singapore. The Associated Press obtained an advance copy of his speech.

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

Perhaps more significantly, officials in Washington said there are 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.

U.S. spy satellites detected signs the North was preparing to transport a long-range missile by train to its northeastern Musudan-ni launch pad, an official at South Korea’s Defense Ministry said today. The official, who spoke on customary condition of anonymity, said the missile was from an armament factory near Pyongyang.

South Korea’s Yonhap news agency said the latest test launch Friday was a surface-to-air missile designed to defend against aircraft or other missile attacks. It said the missile was believed to be a modified version of the Russian SA-5.

The nuclear test and flurry of missile launches, coupled with the rhetoric from Pyongyang that it won’t honor a 1953 truce ending the fighting in the Korean War, have raised tensions in the region and 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.

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