WASHINGTON -- Iran is stepping up its military assistance to President Bashar al-Assad of Syria, and the supplies have strengthened his belief that he can prevail in his struggle with the opposition, a senior State Department official said Wednesday.
"They are plussing up their assistance," said Robert S. Ford, the American ambassador to Syria, referring to Iran. "They are plussing up their people on the ground. They are plussing up what they sending in."
The continued support from Iran, Russia and Hezbollah, the Lebanese militant group, have had an important influence on Mr. Assad's calculations, Mr. Ford said.
"Today, he still thinks he can win militarily," Mr. Ford noted in testimony to the House Committee on Foreign Affairs.
Mr. Ford left Syria over a year ago, shortly before the American Embassy in Damascus was closed because of security concerns, but he continues to hold the title of ambassador and has been the senior State Department official dealing with Syrian opposition groups.
In recent weeks, the Obama administration has shifted its public statements on Syria in an effort to persuade Mr. Assad that he must give up power and agree to a political transition. It has stated that it supports efforts by Arab states like Saudi Arabia and Qatar to provide arms to moderate elements of the resistance. It has also indicated that it backs British and French efforts to persuade the European Union to lift its embargo on the shipment of arms to Syria.
"President Obama has made it clear that the United States does not stand in the way of other countries that have made a decision to provide arms, whether it's France or Britain or others," Secretary of State John Kerry said this week. "He believes that we need to change President Assad's calculation.
While the Obama administration has declined to send arms itself, it has pledged food rations and medical supplies to the Free Syrian Army, as the opposition's main military force is known. The Central Intelligence Agency has also been training rebels in Jordan.
But Iran's aid to Mr. Assad has stiffened his resolve, Mr. Ford said.
The Iranian involvement is so substantial, Mr. Ford said, that Iran's paramilitary Quds Force had "lost a general" in the recent fighting in Syria, among other personnel. Iraqi Shiite extremists have also crossed into Syria to fight on behalf of the government, he noted.
Another major problem is Iraq, which has resisted American demands for inspections of Iranian flights that have been ferrying arms to Damascus, the Syrian capital. The United States has raised its concerns with Prime Minister Nuri Kamal al-Maliki and Mr. Maliki's national security adviser, Faleh al-Fayad, who met last month with Vice President Joseph R. Biden Jr. and Mr. Obama's deputy national security adviser, Tony Blinken.
"We have had very direct conversations with the Iraqis," Mr. Ford said.
Mr. Ford said that the United States had told the Iraqis that the Iranian shipments were prolonging the two-year-old conflict, hampering the prospects for a negotiated political transition and increasing the odds that Sunni extremists will grab a share of the power in Syria.
"We want the Iraqi government to understand that it has no interest in having an extremist government in Syria," Mr. Ford said.
"We need all the pressure we can get on the Iraqis."
Addressing reports that chemical weapons might have been used in Syria on Tuesday, Mr. Ford said the United States still did not have proof that they had been employed.
But he emphasized that the United States was still investigating the reports. The Obama administration has said the rebels did not use chemical weapons, but it left open the possibility that Mr. Assad's government used such weapons in a limited way and then sought to blame the resistance.
"We take these reports and these possibilities very seriously, and we are using all of our available means to determine what happened," Mr. Ford said. "There are reports about them being used both in the north and in the Damascus suburbs, the eastern suburbs of Damascus."
He reinforced the Obama administration's position that the use of chemical weapons was a "red line" for the White House and that it would have serious consequences, but he declined to say what those would be.
This article originally appeared in The New York Times.