Correction: An earlier version of this article incorrectly referred to Oman as a Sunni Arab nation. The majority of Omanis are Ibadi Muslims, not Sunnis. This version has been updated.

Iran has sent soldiers to Syria to fight alongside forces loyal to President Bashar al-Assad and those of the Lebanon-based Hezbollah militia, a senior State Department official said Tuesday.

An unknown number of Iranians are fighting in Syria, the official said, citing accounts from members of the opposition Free Syrian Army, which is backed by the United States. The official spoke on the condition of anonymity to preview a strategy session that Secretary of State John F. Kerry is to hold Wednesday with key supporters of the Syrian opposition.

Rebel forces have alleged for weeks that Iran is sending trained fighters to Syria, and the Iran-backed Hezbollah has said baldly that it will not let Assad fall.

But with the British, French and American governments considering providing arms to the Syrian opposition on a scale not yet seen in the civil war, the U.S. official’s allegation was a tacit acknowledgment that the two-year-old Syrian conflict has become a regional war and a de facto U.S. proxy fight with Iran.

“This is an important thing to note: the direct implication of foreigners fighting on Syrian soil now for the regime,” the official said.

Kerry is in the Middle East this week to foster political talks between Assad’s resurgent regime and the embattled rebels and to inaugurate a new round of peace talks between Israel and the Palestinians.

The State Department official said the Syrian opposition, which is badly split, has not finalized its representative to the talks in Amman, Jordan, on Wednesday. The Amman session is intended to align strategies ahead of a larger conference in Switzerland that would bring together the Russian- and Iranian-backed Assad regime and the Western-backed rebels.

Russia appears to be hedging its bets, as the U.S. official acknowledged Tuesday. Assad’s forces are being resupplied from somewhere, the official said, and not all of the armaments can be explained away as part of a continuation of weapons contracts that predate the conflict.

Kerry and Russian Foreign Minister Sergei Lavrov agreed two weeks ago to jointly lobby the opposition and Assad’s government to sit down for negotiations. The goal would be a transitional government with members chosen by mutual consent. The United States says that would mean Assad’s eventual exit; Russia says not necessarily.

Kerry stopped in Oman on Tuesday to solidify a partnership with a nation that has friendly relations with both Iran and the United States. He was readying plans with Sultan Qaboos bin Said for Oman’s purchase of an estimated $2.1 billion air-defense system. The Raytheon-built system is part of a coordinated, U.S.-led detection and defense network intended to counter Iran’s sophisticated missile systems.

The State Department official would not say whether Iran was welcome at the Syria conference in Geneva, tentatively set for June.

In Washington on Tuesday, the Senate Foreign Relations Committee passed legislation authorizing President Obama to send weapons to vetted Syrian opposition groups. Although the administration has not decided whether to provide lethal aid and does not need congressional approval to do so, the measure would strengthen Obama’s case against those lawmakers who disapprove of stepped-up U.S. involvement in Syria.

The bill, co-sponsored by Sen. Robert Menendez (D-N.J.), the committee chairman, and Sen. Bob Corker (R-Tenn.), the ranking minority member, also creates a $250 million annual transition fund — from reprogrammed, not newly appropriated, money — to help the civilian opposition preserve government institutions and strengthens sanctions against anyone providing arms or selling oil to Assad.

Menendez acknowledged concerns that U.S. weapons could fall into the hands of Islamist extremists fighting on the side of the opposition. But, he said, “if we stand aside and do nothing,” such worries “will become self-fulfilling prophecy.”

The bill, which passed the committee on a bipartisan 15 to 3 vote, still requires approval by the entire Senate and by the House, which has no companion version pending.

Karen DeYoung in Washington contributed to this report.