The United States demanded a cut off of Iranian oil exports to major importers like China and India who had been granted exemptions from sanctions, sending crude prices to six-month highs on fears the U.S. action could lead to a supply crunch.

Secretary of State Mike Pompeo, in a briefing on Monday, said the aim was to halt Iran’s exports entirely, as it continues to pressure Tehran to curtail its nuclear program, ballistic missile tests and support for conflicts in Syria and Yemen.

The Trump administration said it was working with top oil exporters Saudi Arabia and the United Arab Emirates to ensure the oil market was “adequately supplied” but the market, already fretting about tight supplies, raised skepticism about whether Riyadh could take a slower approach in boosting exports.