After weeks of soaring tensions over US-South Korean war games that ended this week, North Korea has toned down its warlike rhetoric in what observers say is a sign that it may now be ready to talk.

Pyongyang had accused the United States of using the two-month joint exercises as a platform for a planned military strike, and reacted furiously to the use of nuclear-capable bombers in the drills.

But after weeks of threats of missile strikes and nuclear war by the North, images of its leader Kim Jong-Un inspecting troops have given way to pictures of his visits to sports events and a health complex with his wife.

"North Korea appears to be taking a breather after its brinkmanship reached fever pitch," said Dongguk University professor Kim Yong-Hyun.

He said Pyongyang could further shift its stance after an upcoming summit in Washington on May 7 between US President Barack Obama and South Korean leader Park Geun-Hye, who took office in February.

"North Korea wants some gestures from Obama. Then it may try to open dialogue with the United States as it did before," he said.

If it fails to achieve its goal, "the regime may go ahead with missile launches or another nuclear test", Kim said.

Tensions have soared on the Korean peninsula since December, when the North test-launched a long-range rocket. In February, it upped the ante once again by conducting its third nuclear test.

While the North has a history of sabre-rattling followed by a cooling-off period in an effort to win concessions from the West, experts say Washington and Seoul may be losing patience.

"The South and the US have become weary of the North's pattern of threats followed by offers of dialogue, and this tactic has lost much of its momentum," said Yun Duk-Min, professor at the Korea National Diplomatic Academy in Seoul.

"The United States will not start any talks with the North that would include the issue of recognition as a nuclear-armed state," he said, referring to a key goal of Kim Jong-Un's regime, along with its demands for a formal peace treaty, an end to UN sanctions and normalised relations.

Experts believe the North will try to use a Korean-American tour operator sentenced this week to 15 years' hard labour for "hostile acts" as a bargaining chip to win concessions from Washington.

The United States called for his "immediate release", saying there had not been transparency in the case.

But the barrage of apocalyptic threats in recent weeks from Pyongyang means that Obama will be reluctant to soften his stance against North Korea, according to experts.

"Obama can't afford politically to make even slightly conciliatory gestures towards Pyongyang after the recent threats fuelled anti-North Korean sentiment in the US," said analyst Paik Hak-Soon of the Sejong Institute in Seoul.

"But if the two leaders do not clearly lay out a coordinated policy stance on the North during their meeting, we will see the same cycle being repeated all over again," he said.

During a trip to Seoul, Beijing and Tokyo last month, US Secretary of State John Kerry repeatedly stressed Washington was ready to talk to Pyongyang -- provided it was serious about reining in its nuclear programme.

But he dismissed as "unacceptable" the North's demands for an end to UN sanctions and a halt to South Korea-US joint military drills.

Leonid Petrov, a North Korea expert at Australian National University, said that as long as Washington maintains its tough stance against the North "there is no chance for improved inter-Korean relations".

"North Korea will continue preparing for war against the US," with nuclear and missile tests, he said.

"Talks are possible and advisable but only between Pyongyang and Washington, which is solely up to the Obama administration and, therefore, unlikely to happen."

Relations between the two Koreas have been further soured by a row over a joint factory park inside the North that was once a rare symbol of cross-border cooperation but now faces the prospect of permanent closure.

Park also appears to be taking an unbending stand against the North, perhaps mindful of Pyongyang's history of testing the South's leaders when they come to power.

"Park is making it clear that she will not soften her stance towards the North just because Pyongyang keeps mounting threats and ratcheting up the tension," said Paik.