The World Trade Organisation (WTO) has launched a probe to decide by the end of this year on whether hefty tariffs on steel imports imposed by the United States violate international trade agreements.

Countries in the 144-member body approved creation of a three-person expert panel - requested by the European Union with support from Japan, China and South Korea - at a meeting of the WTO's Dispute Settlement Body (DSB).

The United States, which insists the measures imposed in March are in line with WTO rules and says it will defend its case vigorously, blocked a first request from the EU at a DSB meeting on May 22.

But under the trade watchdog's dispute regulations, it could not do so a second time and the decision to set up the panel was automatic and taken without debate.

The steel row - which has set the United States against most other producer countries - has stoked accusations that the administration of President George W.Bush is pursuing a unilateralist course to serve his own domestic political aims.

Those charges have been intensified with Bush's approval of a farm bill pumping huge new subsidies into US agro-businesses active on global markets and with US refusal to commit to new targets aimed at reducing global warming and others on promoting world sustainable development.

Members of the WTO panel - which is expected to fold the Chinese, Japanese and South Korean cases into that of the EU - are likely to be selected within the next month and under DSB regulations would normally have up to six months to hand down a ruling.

But this time-frame can be extended, and diplomats say they doubt that the panel could stick to the deadline in such a complicated - and politically sensitive - dispute.

Even when it is handed down, the panel's ruling can be challenged by either side, moving the case to the WTO s quasi-judicial Appellate Body.

Appeals hearings and other procedures would mean that it could be late next year before there is a final decision.

The EU and several other countries - which also include Brazil, Norway and Switzerland - argue that the tariffs of up to 30 per cent on a wide range of steel products are in violation of a range of WTO pacts, including the Safeguards Agreement invoked by Bush in announcing them in early March.

Brussels and Tokyo say that the agreement allows them to impose retaliatory tariffs on other US goods from June 18 unless Washington offers them compensation for their losses on steel sales to the United States.

But they have indicated that they may hold off to allow more time for a US move in that direction.