The U.S. and Iraq have been discussing possible U.S. airstrikes in support of a stalled Iraqi ground offensive against a dug-in Islamic State force in the northern city of Tikrit, U.S. officials said Wednesday.

The prospect of U.S. airstrikes in Tikrit raises highly sensitive questions about participating in an Iraqi campaign that has been spearheaded by Iranian-backed Iraqi Shiite militias.

Iran has provided artillery and other weaponry for the Tikrit battle, and senior Iranian advisers have helped Iraq coordinate the offensive. Iraq pointedly did not request U.S. air support when it launched the offensive in early March.

Recently, the offensive has lost momentum. Col. Steve Warren, a Pentagon spokesman, said Wednesday the Iraqi forces have encircled Tikrit but not yet made significant inroads into the heavily defended city limits.

"They are stalled," he said.

The U.S. has said it is not coordinating any military actions with the Iranians.

Two U.S. officials confirmed the talks about possible U.S. airstrikes. They were not authorized to discuss the matter publicly on the record and so were granted anonymity.

Warren, the Pentagon spokesman, said Wednesday that at Baghdad's request the U.S. has begun aerial surveillance over Tikrit and is sharing the collected intelligence with the Iraqi government.

Warren would not discuss the prospect of U.S. airstrikes in Tikrit nor comment on the possibility that U.S. aerial surveillance of the battlefield in recent days is aimed at identifying targets for potential follow-on U.S. airstrikes.

"I am not going to get into details of future operations," Warren said.

The spokesman said the Iraqis, who initially said they did not need American air power in Tikrit and were satisfied with their partnership with Iran, are discovering how difficult it can be to carry out ground operations in an urban area.

"I think it's important that the Iraqis understand that what would be most helpful to them is a reliable partner in this fight against ISIL," Warren said.

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Associated Press writer Lolita C. Baldor contributed to this report.