The pressure to change the United States' approach comes one year after President Obama said at a White House news conference that the U.S. did not yet have a fully developed strategy for dealing with the
European officials have grown pessimistic about the U.S. strategy amid the daily influx of tens of thousands of migrants, many fleeing the Syrian fighting, and the failure of the U.S.-led coalition to dislodge Islamic State, the Sunni extremist group also known as ISIS or ISIL.
"We are not winning this at the moment," a senior Western diplomat told reporters in Washington on Wednesday. "We need to redouble our efforts collectively to see whether there isn't more that we can do to solve these dual problems: the humanitarian crisis and the growth of ISIL terrorism."
The advances by Islamic State and other opposition groups have rattled the government of Syrian President
Russia has sent a detachment of marines to Syria, U.S. officials say, adding to an escalating military presence that includes increased flights of Russian cargo planes into an airport in western Syria, stepped-up deliveries of armored fighting vehicles and other weapons and the construction of housing for further Russian forces.
U.S. officials and allies do not yet know what Russia's intentions are in Syria, but fear its involvement could extend Assad's military capabilities and prolong a civil war that has killed more than 300,000 people over the last four years.
Russian Foreign Ministry spokeswoman Maria Zakharova called the U.S. claims of a Russian military buildup in Syria "a strange hysteria" and noted that the Kremlin has a long history of military aid to Damascus.
"Russia has never made a secret of its military-technical cooperation with the Syrian Arab Republic," she said in a statement posted on the ministry website and carried by state-run media. "Our country has long supplied Syria with weapons and hardware under bilateral contracts."
"Coordinated actions with the Syrian armed forces should be a major element in consolidating efforts to counter terrorism," the statement added.
France and Britain have also increased their military involvement in Syria.
On Monday, the British government disclosed that it had carried out a drone strike in Syria that killed two British nationals fighting with Islamic State. British Defense Secretary Michael Fallon said the government would not hesitate to launch more drone strikes in Syria if necessary.
That same day, French President
British Prime Minister David Cameron told the House of Commons on Wednesday that he wanted a new approach to driving both Assad and Islamic State out of Syria. "Some of that will require not just spending money, not just aid, not just diplomacy, but it will on occasion require hard military force," Cameron said.
But Germany's foreign minister criticized the military moves, saying the increased British and French sorties would derail diplomatic efforts to end the conflict.
The concerns about Islamic State's gains in Syria come as U.S. intelligence officials see signs of the group expanding its presence in Libya and in Afghanistan, where, one U.S. official said, it has begun to shift from being a distant, inspirational force to one that can carry out attacks.
The expansion contrasts with public statements by administration officials, who have described the militants as largely contained, on the defensive and even losing ground in Iraq.
This summer, the Pentagon's inspector general launched an investigation of whether U.S. Central Command, which oversees military operations in the Middle East, had altered military intelligence assessments to make the campaign seem more successful, officials said.
"Almost everything that has been done in the past year has been almost inconsequential," Bruce Hoffman, a terrorism expert at Georgetown University, said in an interview. "We kill their leaders, but they just continue to seize more territory and slaves and terrorize more of the indigenous population."
Obama has been reluctant to expand the U.S. military role in Syria beyond airstrikes and training of small numbers of fighters associated with what the U.S. sees as moderate groups opposed to Assad.
Dozens of opposition fighters trained by the U.S. military and the CIA to fight Islamic State have been captured or killed. Islamic State has made gains in Iraq as well, taking control of 80% of the oil fields and refineries in the town of Baiji at the southern edge of its current territory despite hundreds of U.S.-led airstrikes and a 10-month Iraqi army campaign to push the militants back.
"We know we need to improve on how we reintroduce Syrian fighters back into the country, and we're currently working on that," said a U.S. official who was not authorized to speak on the record about the matter. "Airstrikes are effective, but airstrikes alone will not win this fight."
The Europeans will try to persuade the U.S. to shift its strategy during meetings planned around the United Nations General Assembly session in New York this month, according to the Western diplomat who spoke with reporters. John R. Allen, a retired Marine Corps general and administration special envoy, is expected to join those meetings.
On Wednesday, Syrian rebel factions seized a key air base in Idlib province in northwestern Syria after a two-year siege, giving them control of the entire province, U.S. officials said. Farther north along the border between Syria and Turkey, Islamic State militants have moved into territory near the city of Aleppo that had been abandoned by another insurgent group, Al Nusra Front, which is an affiliate of Al Qaeda.
Control of that territory is particularly valuable because it can be reached by new recruits from abroad.