North Korea said it would strengthen its nuclear deterrent following President Barack Obama's "dangerous" Asian tour, and would not rule out another atomic test.

There are concerns the North is preparing to conduct its fourth atomic detonation, with recent satellite images showing stepped-up activity at its main nuclear test site.

Mr Obama's tour ended today in the Philippines and took in Japan, South Korea and Malaysia.

A Pyongyang foreign ministry spokesman told state-run Korean Central News Agency that the tour "was a dangerous one as it was aimed to bring dark clouds of more acute confrontation and nuclear arms race to Asia.”

Pyongyang would bolster its nuclear deterrent "now that the US brings the dark clouds of a nuclear war to hang over the DPRK (North Korea)", the spokesman said.

"There is no statute of limitations to the DPRK's declaration that it will not rule out a new form of nuclear test clarified by it in the March 30 statement," he added.

The "new form" of nuclear test could perhaps lay the stage for a test based on new uranium-enrichment technology, analysts have said.

Mr Obama's tour "was designed for undisguised confrontation to retain a tighter grip on allies of the US and encircle and contain its rivals in Eurasia, pursuant to the US Asia-Pacific strategy for domination and scenario for aggression from A to Z," the spokesman added.

While in Seoul, Mr Obama had angered the North by demanding that it abandon its nuclear weapons programme, and by threatening tougher sanctions if it went ahead with another test.

"North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation," he told American troops based in Seoul.

The foreign ministry spokesman said the comments showed that US hostility towards the North remains unchanged, and that it is still trying to topple the state by force.

Meanwhile, South Korea's military said the North had started firing drills near a disputed sea border.

It comes one month after a similar exercise triggered the exchange of hundreds of artillery shells across the disputed boundary.

The firing began at about 2pm local time (6am Irish time) and no rounds appeared to have landed south of the disputed Northern Limit Line border, a military official said.

"The North notified us there would be live-fire drills today north of the [border] near Yeonpyeong and Baengnyeong islands," a defence ministry spokesman said.

"Our military is fully prepared," the spokesman said, adding that South Korean fishing vessels had already been told to withdraw from the area.

North Korea carried out a similar drill on 31 March during which a number of shells dropped into South Korean territorial waters.

The South responded, and the two rivals traded hundreds of rounds of live artillery fire, forcing South Korean islanders to take shelter.

The exchange was limited to untargeted shelling into the sea, but fuelled tensions that had already risen after North Korea threatened to carry out a nuclear test.