The UN's special envoy on Western Sahara should engage with the "central issues" surrounding the disputed territory, Spain's chief diplomat warned in the Moroccan capital Wednesday.

"It would be better for the special envoy dealing with the (Western Sahara) to engage with the central, and not fringe, issues," Jose Manuel Garcia-Margallo said at a joint press conference with Moroccan counterpart Saad Eddine Othmani, whom he had met earlier for talks.

It is the second rebuke for envoy Christopher Ross in as many months after Morocco officially withdrew its confidence in the veteran US diplomat in May, criticising his work in the Western Sahara as "unbalanced and biased".

The Moroccan attack came after the last annual UN report on Western Sahara said its operations had been "undermined" by Moroccan tactics.

Western Sahara, a former Spanish colony annexed by Morocco in 1975, is at the centre of conflict between the Algerian-backed Polisario Front and Rabat, which is willing to accord broad autonomy, but not independence.

UN efforts, led by Ross since 2009, to arrange a negotiated solution have been deadlocked for years.

Spain would continue to examine solutions to the conflict, Garcia-Margallo assured on Wednesday.

"We want a stable, peaceful and lasting solution to this conflict, within the framework of UN Security Council resolutions," he said.