U.N. Secretary General Ban Ki-moon on Tuesday described North Korea's boast that it has missiles capable of reaching the U.S. mainland as "alarming" and warned it would increase tension in the region.

Ban, a former foreign minister of South Korea, admitted he had no way of knowing if the claim issued by North Korea earlier on Tuesday was credible.

Three North Korean soldiers have defected to South Korea across the heavily-fortified border since August, media reports said Tuesday, in an apparent embarrassment to Pyongyang's new regime.

The rare defections -- involving one officer who shot dead two superiors before defecting on Saturday -- prompted the North to launch special probes on soldiers stationed at the border, Yonhap news agency and other media said.

North Korea said Tuesday it possessed "strategic rocket forces" capable of striking the U.S. mainland, as it responded to a new U.S.-South Korean deal to extend the range of the South's missile systems.

In a series of bulletins released on the official Korean Central News Agency (KCNA), the spokesman of the National Defense Commission also said Pyongyang was ready to match any enemy, "nuclear for nuclear, missile for missile".

A North Korean soldier defected to the South on Saturday through the heavily militarized border, saying he shot dead two superior officers in the process, the South Korean military said.

It is only the fourth such defection reported in the last 10 years, with none of the previous incidents involving fatal shootings, and could raise already heightened tensions ahead of the South's presidential election in December.

President Lee Myung-Bak on Thursday said South Korea posed no security threat to North Korea even as he urged parliament to get behind reforms aimed at modernizing the South's military.

In a parliamentary budget speech, Lee also called on the communist state to give up its pursuit of missile and nuclear programs and instead focus on the needs of its impoverished population.

North Korea has dropped thousands of propaganda leaflets attacking South Korea across their heavily militarized border for the second time this year, the South's defense ministry said Tuesday.

South Korean soldiers have collected about 17,000 leaflets, which were floated by balloon over the frontier on Saturday, a ministry spokesman said.

Iraq on Friday denied permission for a North Korean aircraft to cross its airspace on its way to Syria over suspicions it would carry arms and advisers there, the Iraqi premier's spokesman said.

"Iraq forbade a North Korean airplane to cross its airspace on suspicion that it is carrying weapons and advisers for the Syrian side," Ali Mussawi told AFP, adding that the plane was supposed to cross on Saturday.

The South Korean navy fired warning shots Friday at half a dozen North Korean fishing vessels that crossed their disputed Yellow Sea border, the defense ministry said.

The six North Korean boats swiftly returned to their side of the border after the incident, a ministry spokesman told Agence France Presse.

North Korea on Thursday lashed out at a U.S. plan to build a new radar base in Japan, saying it would escalate an arms race and leave Pyongyang no option but to develop "more powerful nuclear weapons".

During a visit to Tokyo on Monday, U.S. Defense Secretary Leon Panetta said an agreement had been reached on deploying another U.S. missile defense radar in Japan.

Washington and Tokyo have agreed to deploy another missile defense radar in Japan, to counter the threat from North Korea, U.S. Defense Secretary Leon Panetta said Monday.

"The purpose of this is to enhance our ability to defend Japan. It's also designed to help forward-deployed U.S. forces," Panetta said of the X-band radar during a joint press conference in Tokyo with his opposite number.