Prime Minister Shinzo Abe has warned that North Korea may be capable of firing a missile loaded with sarin nerve gas towards Japan.

Mr Abe's comments on Thursday come amid looming concern about another missile or nuclear test by the communist country.

Responding to a question about Japan's readiness at a time of increased tension, Mr Abe told a parliamentary panel on national security and diplomacy: "There is a possibility that North Korea already has a capability to deliver missiles with sarin as warheads."

Members of a Japanese doomsday cult killed 12 people and made thousands ill in 1995 in simultaneous attacks with sarin nerve gas on five Tokyo rush-hour subway trains.

Citing Syria where dozens of people died recently in an alleged sarin nerve gas attack, Mr Abe said Japan should take the example seriously, stressing the need to strengthen its deterrence against the North.

North Korea, which is not a signatory to the international Chemical Weapons Convention, has been producing chemical weapons since the 1980s and is now estimated to have as many as 5,000 tons, according to a South Korean defence white paper. Its stockpile reportedly has 25 types of agents, including sarin.

Experts say if North Korea were to attack South Korea, it would likely target Seoul's defences with chemical and biological weapons dropped from aircraft or delivered via missiles, artillery and grenades.

Japan, under its postwar constitution, has limited the role of its military to self-defence only and relied on the US for offensive and nuclear capability. But recently, Mr Abe's ruling party has proposed that Japan should bolster its missile defence, including upgrading the capability to shoot down an enemy missile and acquiring the capacity to attack the base it was fired from.

A US Navy aircraft carrier is heading toward the Korean Peninsula as Pyongyang prepares for the 105th anniversary of the birth of its founder, Kim il-Sung, this weekend.

In 2012, the regime tried but failed to launch a long-range rocket carrying a satellite to mark the date, and tested a newly developed intermediate-range missile last year.

A Washington-based think tank that monitors North Korea, 38 North, said satellite images taken on Wednesday showed continued activity around the Punggye-ri nuclear test site on the east coast that suggested it was ready for a new test.

And the state-funded US site Voice of America said US government sources told it the North had placed a device in a tunnel at Punggye-ri.

But on Thursday South Korean officials said there were no new signs to indicate a North Korean nuclear test was more likely, though they also said the North has maintained a state of readiness to conduct such a test at any time.