SEOUL, South Korea — North Korea vowed to retaliate if punitive U.N. sanctions are imposed for its apparent nuclear test, and U.S. officials said there are new signs 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 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.

A South Korean newspaper reported today that U.S. spy satellites detected signs the North was preparing to transport a long-range missile from an armament factory near Pyongyang to its northeastern Musudan-ni launch pad. The mass-circulation Dong-a Ilbo paper, citing an unidentified source in Washington, said it would take about 15 days for the North to move the missile to the pad by train.

South Korea’s Defense Ministry said it could not confirm the report. But a ministry official said the North manufactured the long-range rocket it fired in April at the factory cited by the newspaper and transported it via train to Musudan-ni. The official spoke on condition of anonymity citing department policy.

The U.S. officials also said an initial U.S. air sampling from near the underground test site was inconclusive. Officials said the initial analysis doesn’t prove the North successfully completed an atomic reaction. At least one more U.S. test is coming.

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.

But officials said the heavily fortified border remains calm, and U.S. Defense Secretary Robert Gates said Washington does 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 North’s 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 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.

“If the U.N. Security Council makes a further provocation, it will be inevitable for us to take further self-defense measures,” the Foreign Ministry said.