TOKYO - Japan said on Monday that North Korea's nuclear test clearly violated the U.N. resolution and Tokyo, already upset with a rocket launch by the isolated country last month, would seek decisive action.

North Korea successfully conducted an underground nuclear test earlier in the day as part of measures to bolster up its nuclear deterrent for self-defense, its official KCNA news agency said. Just hours later, it fired a short-range missile, Yonhap news agency reported.

The U.N. Security Council's resolution 1718 strongly asks North Korea not to conduct nuclear tests. So if North Korea did conduct the nuclear test, that would be a clear violation of the U.N. Security Council resolution and would be unacceptable, Chief Cabinet Secretary Takeo Kawamura told reporters.

While we are cooperating with relevant countries, we will take decisive action, including (seeking action) at the U.N. Security Council, Kawamura, the top government spokesman, added.

The council will meet on Monday to discuss the reclusive country's nuclear test, Russia's U.N. ambassador was quoted by Itar-Tass news agency.

The yen, which showed little immediate reaction to news of the nuclear test, slipped against the dollar and the euro after a Yonhap report that North Korea had also fired a short-range missile on Monday.

In April, North Korea fired what was widely seen as a long-range ballistic missile that flew over northern Japan, prompting Tokyo to extend additional economic sanctions on the communist state and to deploy missile interceptors to the area.

In the wake of last month's rocket launch, Japan had called for a fresh, legally binding resolution by the U.N. Security Council to declare Pyongyang in violation of a previous resolution banning the firing of ballistic missiles.

But with Russia and China opposing the move, the council issued a presidential statement instead, which condemned the rocket launch.

Japanese Prime Minister Taro Aso may try to take a tough stance on North Korea ahead of an election that must be held by October, in which Aso's long-ruling Liberal Democratic Party (LDP) could lose out to the main opposition party.

Kazuo Kitagawa, secretary-general of the LDP's junior coalition party New Komeito, told reporters: Not only the government but parliament must think about this. It is a matter of course that we should think about new sanctions.

North Korea's rocket launch last month, seen in the United States, South Korea and Japan as a thinly disguised long-range missile test, has already breathed new life into a debate over whether Japan's military should acquire the capability to make pre-emptive strikes against enemy bases.

A Japanese ruling party panel is to propose that pre-emptive strikes against enemy bases be allowed despite the country's pacifist constitution, Kyodo news agency said on Monday.

While some lawmakers have called for strike capability, Defense Minister Yasukazu Hamada is among those are cautious about the prospect, though the government's stance is that such strikes should be allowed if an attack were certain to take place.