South Korea and the United States have agreed on a two-year extension to a civilian nuclear pact that Seoul wants amended to allow it to produce its own nuclear fuel, officials said Wednesday.

The current pact, signed in 1974, had been due to expire next year. The extension was agreed to allow more negotiations on the heated topic of allowing the South to reprocess spent fuel rods.

"The two sides reached a temporary agreement on extending the current accord by two years on the grounds that they need more time," Foreign Ministry spokesman Cho Tai-Young told reporters.

South Korea argues that it needs to produce its own nuclear fuel to feed the 23 reactors that provide one-third of its energy needs and to deplete stockpiles of spent fuel rods which it says are reaching full capacity.

The United States has refused on proliferation grounds, as reprocessing creates stockpiles of separated plutonium that can then be enriched to weapons grade.

The delay on settling the controversial issue removes a main item of dispute between the allies before a visit to Washington by South Korean President Park Geun-Hye on May 7-8.

In Washington, State Department spokesman Patrick Ventrell said only that the United States wanted to be "careful" as the future agreement would be in place for years to come.

"Both sides decided that seeking an extension was appropriate in order to allow sufficient time to negotiate this complex and technical agreement," Ventrell told reporters.

The State Department said that the two countries would hold their next talks on the agreement in June, with further rounds held every three months.

South Korea has proposed pyro-processing, a new technique considered less conducive to proliferation as it leaves separated plutonium mixed with safer fissile materials.

The issue of allowing South Korea to produce its own nuclear fuel has become more vexed in the light of North Korea's advancing nuclear weapons programme.

This has led to growing calls from an influential minority in South Korea for the country to have its own deterrent, rather than to keep relying on the US nuclear umbrella.