Following months of unrest in Zimbabwe, Jack Straw was heading to Nigeria on Wednesday for Commonwealth talks aimed at resolving disputes over land reform.

Speaking as he headed to the meeting of seven Commonwealth foreign ministers, the foreign secretary said he would approach discussions in a "constructive" but "realistic" manner.The meeting comes as Zimbabwe's president, Robert Mugabe, continues to quash his critics and flout international law. Despite calls for tough action against Zimbabwe, Commonwealth sources have in recent days argued that dialogue must be maintained with the controversial African leader.

As senior Tories called for the suspension of Zimbabwe from the Commonwealth, Don McKinnon, its secretary general, said it remained vital that links were maintained with Mugabe's administration.

"We've had calls for a long time for us to suspend Zimbabwe from the Commonwealth. But at the same time Commonwealth governments want us to keep engaged, want us to try to help where we can and certainly to move them away from the course of action they are on,'' he said on Wednesday.

However, the shadow foreign secretary, Francis Maude, warned that Zimbabwe's place round the Commonwealth table appeared to give "a cloak of completely undeserved respectability to the Mugabe regime".

"The time has come really when the Commonwealth has got to blow the whistle in a really serious way. I don't think it can be allowed to carry on," he told the Today programme.

Maude's comments were echoed by pro-democracy campaigners in Zimbabwe who called for Mugabe to be ostracised by international organisations.

However, McKinnon said ejecting Zimbabwe would run counter to the Commonwealth's approach.

"The Commonwealth doesn't act in a precipitous way. It just doesn't throw someone out because it feels like it or because someone in the British media demands we do it. It does work in a very steady, deliberative way," he said.

He added there was ambiguity over the powers of the Commonwealth to act against Zimbabwe in the circumstances.

"We do not have any battalions at our disposal. We don't have the authority just to go in and clean up this. We can only do this by persuasion and we can only do this while we are firmly attached to this as part of the membership," he said.

And he warned that breaking diplomatic links with Mugabe would have proved counter-productive. "If they were thrown out of the Commonwealth 18 months ago, we would have absolutely no contact, no influence whatsoever," said McKinnon.