RABAT (Reuters) - Morocco's king said he will offer no more than autonomy for the disputed Western Sahara, a few days after United Nations chief called for "true negotiations" to end the four-decade deadlock over the region.

Morocco has controlled most of Western Sahara since 1975 and claims the sparsely populated stretch of desert, which has offshore fishing, phosphate reserves and oilfield potential, as its own territory.

However, the Algeria-backed Polisario Front seeks independence and a United Nations mission was formed more than 20 years ago ahead of an expected referendum on Western Sahara's political future which has never taken place.

U.N. special envoy to Western Sahara Christopher Ross has intensified visits to the region and Europe recently to facilitate negotiations without preconditions and in good faith, Secretary-General Ban Ki-moon said in a statement last week.

"This initiative is the maximum Morocco can offer," Morocco's King Mohamed said, referring to the autonomy plan for the region. "Its implementation depends on reaching a final political agreement under the backing of the United Nations."

The king was speaking late on Friday in a televised speech commemorating the 40th anniversary of the Green March day, when thousands of Moroccans marched on Western Sahara.

"Morocco refuses any adventure with an uncertain result and that could be potentially dangerous," he said.

The Western Sahara dispute returned to the headlines last month when Morocco said it was considering a boycott of Swedish companies operating in the North African kingdom because of Sweden's position on the conflict.

The government said Sweden has been campaigning to boycott products from Western Sahara and international companies with a presence there.

"Whoever wants to boycott Moroccan products is free to do so, but they should assume the consequences of their decisions," the king said.

Sweden and other Scandinavian countries have backed Western Saharan self-determination, while France and Spain have been accused by activists and human rights organizations of supporting the Moroccan line.

Polisario's planned Sahrawi Republic (SADR) was recognized by some countries, mainly from the African Union, but none of the Western powers recognized it.

Morocco said it will revive the region through investment, including a new roads program and an international airport serving the rest of Africa. It has called for Moroccan and foreign investors to seize opportunities there.

While rights groups such as Amnesty International accuse Morocco of continuing to use excessive force against activists and repressing political freedom in Western Sahara, Rabat invests heavily there, hoping to calm social unrest and dissuade independence claims.