US Deputy Defense Secretary Ashton Carter on Monday promised to provide South Korea with every military resource under the US nuclear umbrella at a time of heightened tensions with North Korea.

Carter was in Seoul on the second leg of a four-nation tour of US allies and partners in Asia including Japan, the Philippines and Indonesia.

"We remain steadfast to our commitment to extended deterrence offered by the US nuclear umbrella," Carter said after talks with South Korean Defence Minister Kim Kwan-Jin.

"We'll ensure all of our resources will be available to our alliance," he was quoted as saying by Yonhap news agency.

Carter's visit came just days after the United States announced it would bolster defences against a possible North Korean missile strike in response to Pyongyang's threat of a "pre-emptive" nuclear attack.

In Washington, the Pentagon made a point of mentioning a recent training flight of a B-52 bomber over South Korea, saying it underlined America's determination to defend Seoul.

On March 8, the B-52, from Andersen Air Force base in Guam, flew over South Korea as part of a military exercise dubbed "Foal Eagle," spokesman George Little told reporters.

"The B-52 Stratofortress can perform a variety of missions including carrying precision-guided conventional or nuclear ordnance," he said.

Little said similar B-52 flights would continue despite budget pressures and that Washington deemed them more important than ever given recent tensions on the Korean peninsula.

Although he acknowledged the training flights were not new, Little said: "We're drawing attention to the fact that we have extended deterrence capabilities that we believe are important to demonstrate in the wake of recent North Korean rhetoric."

The United States unveiled plans on Friday to bolster missile defenses in light of the threat posed by North Korea.

Defense Secretary Chuck Hagel announced that 14 more interceptors would be stationed in Alaska, increasing by almost half the 30 already deployed along the California and Alaska coastlines.

Hagel said the defence upgrade was designed to "stay ahead of the threat" from North Korea, which is still believed to be years from having a missile capable of carrying a nuclear warhead to the continental United States.

Military tensions on the Korean peninsula have escalated dramatically since the North conducted its third nuclear test last month.

Pyongyang responded to the subsequent UN sanctions with threats of "all-out war" backed by nuclear weapons.

Some lawmakers in Seoul have suggested it is time for South Korea to develop its own nuclear deterrent, rather than relying on the US umbrella.