“They are working earnestly to keep Assad in power,” he said, explaining that in addition to arms and scores of Quds Force trainers and Iranian intelligence agents, Iran is providing the Syrian security services with electronic eavesdropping equipment “to try and pick up where the opposition networks are.”

In early January, American intelligence officials said, the Quds Force commander, Qassim Suleimani, visited Damascus, Syria, raising suspicions that Iran was advising Mr. Assad on how to quash the uprising. “What we’re seeing is a much more aggressive Iranian effort to become involved in a number of areas and activities,” President Obama’s counterterrorism adviser, John O. Brennan, said in a recent interview.

The authorities in Azerbaijan announced Wednesday that they had arrested 22 Azeri citizens suspected of spying for Iran’s Revolutionary Guards and plotting to attack the United States and Israeli Embassies and the British oil company BP, according to Reuters, citing the country’s National Security Ministry.

Analysts say Yemen could be highly useful in any effort by Iran to retaliate against an Israeli attack on Iranian nuclear facilities. The country’s longtime president, Ali Abdullah Saleh, formally stepped down earlier this year after a year of widespread protests and violence, but Yemen remains highly volatile, with its political elite divided and much of the country outside the control of the government. Militants linked to Al Qaeda continue to battle the Yemeni military in the south, and much of the north is under the control of the Houthi rebels.

The Houthi rebels are based just across the border from Saudi Arabia, and they practice a quasi-Shiite form of Islam that makes them natural Iranian allies. Skilled guerrilla fighters, they fought a short war with Saudi Arabia in 2009, and could presumably be used as an Iranian proxy force. “Iran is hoping to use Yemen as a pressure point against Saudi Arabia and all the countries in the Arab Gulf,” said Yahya al-Jifri, a leader of Al Rabita, one of Yemen’s independent political parties.

A Houthi spokesman, Yahya al-Houthi, denied that the movement had received any Iranian weapons, training or money, and added that the accusation was an old one leveled by the United States and Saudi Arabia.

Many Yemeni political and tribal figures dismiss any Iranian military support as insignificant, noting that the Houthis have plenty of weapons, and that Saudi Arabia has been supplying Yemeni factions with arms for decades. Some add that any substantial shipments of arms across inland Yemen would have left a clear trail of evidence.