South Korea gave a guarded response Friday to a Japanese deal to ease sanctions against North Korea, stressing the need to maintain a united front against Pyongyang's nuclear weapons programme.

Japanese Prime Minister Shinzo Abe announced Thursday that Tokyo would relax some sanctions if North Korea delivers on a pledge to reinvestigate the cases of Japanese nationals kidnapped to train spies.

The breakthrough followed days of talks between the two sides in Sweden, and marked the most significant engagement between Pyongyang and the outside world in many months.

In a statement, South Korea's foreign ministry said Seoul appreciated the emotional pull for Tokyo of the long-standing and highly-charged kidnapping issue.

"The government, from a humanitarian standpoint, understands Japan's stance concerning the issue of Japanese abductees," the foreign ministry said in a statement.

But it also stressed that no cracks should be seen to appear in the united stance between Japan, South Korea and the United States on the need for North Korea to end its nuclear weapons programme.

"We will continue watching the Japan-North Korea consultations in this context," it said.

The sanctions Tokyo is considering easing are not of major significance, involving travel restrictions and North Korea-registered vessels entering Japanese ports for humanitarian purposes.

But the deal comes at a time when others in the international community are pushing for North Korea's economic and diplomatic isolation to be deepened rather than relaxed.

And it will do little to improve ties between Tokyo and Seoul which are already at their lowest ebb for years over a series of historic bilateral disputes.

US President Barack Obama sought to smooth things over by hosting a trilateral summit with Abe and South Korean President Park Geun-Hye in March, but relations remain distinctly frosty.