HONOLULU — Fortifying one of its key allies in the Persian Gulf, the Obama administration announced a weapons deal with Saudi Arabia on Thursday, saying it had agreed to sell F-15 fighter jets valued at nearly $30 billion to the Royal Saudi Air Force.

The agreement, and the administration’s parallel plans to press ahead with a nearly $11 billion arms deal for Iraq, despite rising political tensions there, is dramatic evidence of its determination to project American military influence in an oil-rich region shadowed by a threat from Iran.

Though the White House said the deal had not been accelerated to respond to threats by Iranian officials in recent days to shut off the Strait of Hormuz, its timing is laden with significance, as tensions with Iran have deepened and the United States has withdrawn its last soldiers from Iraq.

“This sale will send a strong message to countries in the region that the United States is committed to stability in the gulf and the broader Middle East,” said Andrew J. Shapiro, the assistant secretary of state for political-military affairs. “It will enhance Saudi Arabia’s ability to deter and defend against external threats to its sovereignty.”