SEOUL, South Korea — North Korea has resumed work at its underground nuclear testing site, defense analysts said, as the country vowed to keep expanding its nuclear arsenal despite the latest United Nations sanctions.

The defense analysts also said that the North’s Sept. 3 nuclear test, which Pyongyang said was of a hydrogen bomb, may have been much more powerful than previously estimated.

In its first official reaction to the sanctions resolution adopted by the United Nations Security Council on Tuesday, North Korea’s Foreign Ministry said on Wednesday that the sanctions would only strengthen the country’s resolve to pursue its nuclear weapons program “at a faster pace without the slightest diversion.”

The sanctions resolution, adopted in response to the nuclear test this month, was the ninth passed by the Security Council since North Korea’s first such test in 2006. If enforced, it would deprive North Korea of 30 percent of its annual fuel imports. It also bans imports of textiles from North Korea, stripping the country of another key source of hard currency.