United Nations ? The U.N. Security Council expressed “grave concern” Thursday about Iran’s seizure of 15 British sailors and marines and called for an early resolution of the escalating dispute, but Iran’s chief international negotiator suggested the captives might be put on trial.

The council’s statement wasn’t as tough as Britain had hoped, though, and the divide seemed to deepen.

As the standoff drove world oil prices to new six-month highs, Turkey, NATO’s only Muslim member, reportedly sought to calm tensions by urging Iran to let a Turkish diplomat meet with the detainees and to free the lone woman among the Britons.

Tensions had seemed to be cooling a day earlier, but after Iran offended leaders by airing a video of the prisoners and Britain touched a nerve in Tehran by seeking U.N. help, positions hardened even more Thursday.

Iran retreated from a pledge by Iranian Foreign Minister Mottaki that the female sailor, Faye Turney, would be released soon. Mottaki then repeated that the matter could be resolved if Britain admitted its sailors mistakenly entered Iranian territorial waters last week.

Britain’s Foreign Office insisted again that the sailors and marines were seized in an Iraqi-controlled area while searching merchant ships under a U.N. mandate and said no admission of error would be made.

With Britain taking its case to the United Nations, Ali Larijani, the top Iranian negotiator in all his country’s foreign dealings, went on Iranian state radio to issue a warning.

He said that if Britain continued its current approach, “this case may face a legal path” – a clear reference to Iran prosecuting the sailors and marines in court. “British leaders have miscalculated this issue,” he said.

Gen. Ali Reza Afshar, Iran’s military chief, blamed the backtracking on releasing the British woman on “wrong behavior” by her government. “The release of a female British soldier has been suspended,” the semiofficial Iranian news agency Mehr said.

The Security Council’s statement was a watered-down version of a stronger draft sought by Britain to “deplore” Iranian actions and urge the immediate release of the prisoners, primarily because Russia and South Africa opposed putting blame on the Tehran regime, diplomats said.

Russia also objected to the council adopting Britain’s position that its sailors were operating in Iraqi waters when they were captured.