U.N. inspectors headed to North Korea's key nuclear reactor Thursday to discuss a long-delayed shutdown of the facility, as the country came under increasing international criticism for launching missile tests this week.

Japanese Prime Minister Shinzo Abe slammed North Korea's communist government over the short-range missile launches, calling them a provocation that defied the United Nations and could destabilize the region.

"We need to seek a harsh response from the international community," Abe said in Tokyo.

• Monitor the nuclear showdown on the Korean Peninsula in FOXNews.com's North Korea Center.

North Korea boosted the urgency in the international stand-off over its nuclear program in October when it conducted its first atomic test explosion. The U.N. Security Council condemned the move and passed a resolution saying North Korea must, among other things, abide by a missile-test moratorium.

"I do not think this will directly affect our security," Abe said of this week's missile testing. "But in any case it is a violation of the U.N. Security Council resolution."

U.S. officials in Washington also criticized the launches, which happened either Tuesday or Wednesday, according to varying reports.

"We expect North Korea to refrain from conducting further provocative ballistic missile launches, activity that is destabilizing to the security of northeast Asia," said Gordon Johndroe, a spokesman for the U.S. National Security Council.

Meanwhile, an International Atomic Energy Agency team traveled from the North Korean capital to the Yongbyon reactor, about 100 kilometers (60 miles) to the northeast.

The 5-megawatt reactor, believed capable of churning out enough plutonium for one atomic bomb per year, is at the center of international efforts to halt North Korea's nuclear program.

The team was invited by North Korea to discuss details of shutting down the reactor, as it pledged under an international accord in February. It is the first IAEA trip to the facility since its monitors were expelled from the country in late 2002.

"We go to see the facilities and continue our discussions in more details," Olli Heinonen, deputy director of the International Atomic Energy Agency, said in footage shot by APTN at his Pyongyang hotel before he departed for the Yongbyon complex.

Heinonen, whose team arrived in Pyongyang on Tuesday, declined to provide details of his discussions with North Korean officials so far and emphasized that the visit to Yongbyon, expected to last into Friday, was not a formal inspection.

"We are here to talk about the verification and monitoring arrangement," Heinonen said.

Asked if the North might begin to shut down the reactor during his visit there, Heinonen told reporters that he and his team will see "what we have on the table" Friday evening.

Plans for a formal inspection of the facility would need approval by the Vienna-based IAEA board of governors, Heinonen said Wednesday.

The North agreed in February to close the reactor in exchange for economic aid and political concessions, under an accord reached in six-party talks also including the U.S., China, Japan, Russia and South Korea.

But the communist nation ignored an April deadline to do so because of a banking dispute with the United States.

That dispute was settled this week after months of delay, and North Korea announced Monday that it would move forward with the disarmament deal.

Japanese Foreign Minister Taro Aso and his Chinese counterpart Yang Jiechi agreed in a phone conversation Thursday that the accord's initial measures should be carried out completely and evenly, Japan's Foreign Ministry said in a statement. The accord's initial phase calls for North Korea to shut the Yongbyon reactor and receive 50,000 tons heavy fuel oil assistance.

South Korea's Unification Ministry said Thursday it has agreed with North Korea to discuss details of the envisioned oil provision, such as how much should be shipped to which ports, in talks on Friday and Saturday at the North Korean border city of Kaesong.

On Wednesday, South Korean news agency Yonhap reported that North Korea tested a short-range missile.

Two officials at the U.S. Defense Department confirmed the report, but said there were three launches and that they took place Tuesday. The discrepancies could not immediately be reconciled.

The missiles were fired within the North's territorial waters, the U.S. officials said, speaking on condition of anonymity because of the sensitivity of the issue.

Such missile tests can spike tensions in the standoff over North Korea's nuclear program. However, it is not believed capable of mounting a nuclear weapon on a missile.

It was the third time in a month that the North test-fired a short-range missile, following launches May 25 and June 7.

Complete coverage is available in FOXNews.com's North Korea Center.