U.S. fighter jets and armed drones scream skyward every few minutes from this sprawling air base in southern Turkey, flying deep into neighboring Syria on bombing runs against
The intensity of the U.S.-led air war has increased sharply since Turkish President
But the Obama administration says Turkey isn't doing enough to close smuggling routes the militants use to move fighters, money and weapons in and out of Syria, especially along a porous 60-mile stretch known as the Manbij Pocket.
"We want them to do more," Defense Secretary
Carter is visiting the region this week to deliver the "do more" message to Turkey and several Sunni Arab allies.
Secretary of State John F. Kerry met Tuesday in Moscow with Russian President Vladimir Putin as part of a broader push to negotiate a political resolution to Syria's civil war, now in its fifth year.
"Together, the United States and Russia have an ability to be able to make a significant difference here," Kerry said in a brief appearance with Putin at the Kremlin.
Washington and Moscow disagree sharply over a future role for Syrian President
Carter's stop here focused attention on the maze of supply lines that go through Turkey for insurgent groups fighting in the multi-sided Syrian war, and the competing motives of those involved in the conflict.
Towns on the Turkish side of the 500-mile border have become supply bases, logistics hubs and medical treatment centers for moderate and extremist Syrian rebel groups, including Islamic State, according to U.S. officials. Thousands of foreign fighters have slipped across the border to join the militant factions.
Turkish officials worry, in turn, about U.S. support for Kurdish militias that do the bulk of the fighting against Islamic State forces in northern Syria. Some are a proxy for the Kurdistan Workers Party, known as the PKK, which has battled for greater autonomy from Turkey for more than three decades. Turkey fears their presence could lead to the formation of a breakaway Kurdish state.
In addition to increasing the pace of airstrikes, the Pentagon recently sent a small contingent of special operations troops into northern Syria to coordinate with Kurdish militias and other opposition groups that are trying to cut off Islamic State supply lines to Raqqah.
Washington accuses the Turkish government of failing to meet its promise to close the border and form a "safe zone" free of Islamic State militants.
A safe zone would also pose problems. Turkey says such an area would require the protection of a coalition-enforced no-fly zone, but at the same time, it is unwilling to supply the necessary ground troops to do so. The U.S. has said it would not provide ground troops to patrol a safe zone.
"There is a clear mismatch of U.S. and Turkish priorities," said Henri J. Barkey, director of the Middle East program at the Woodrow Wilson International Center for Scholars in Washington.
Some Kurds and other critics have accused Erdogan's government of providing direct support to Islamic State militants and allowing them to cross into Syria to fight the Kurdish militiamen.
Turkey denies the charge and points to Incirlik, which has hosted U.S. Air Force planes since the 1950s, as evidence of its commitment to the fight.
Nearly 60 coalition warplanes now operate here, up from 15 in early September, and more are expected. Most are American, including a dozen A-10 attack aircraft and a dozen F-15 fighter jets added last month. The increase has led to another problem.
"Incirlik is a big base, but there's only so much concrete to park aircraft," said Col. Sean McCarthy, an A-10 pilot and commander. "It's crowded."
Despite being a fellow member of the North Atlantic Treaty Organization, Turkey resisted letting the Obama administration expand operations from Incirlik for nearly a year after the U.S.-led air war began in Syria in 2014.
Turkey portrayed the shift in September as a signal of its resolve to fight Islamic terrorism, an umbrella under which Erdogan includes the PKK. Turkish warplanes participating in the effort have targeted PKK forces ahead of — and sometimes instead of — Islamic State positions.
"This was a way for Turkey to increase its strategic importance to the U.S. while making it easier to crack down more aggressively on the PKK without too much international reaction," Nigar Goksel, senior analyst for the nonpartisan International Crisis Group, said in a telephone interview from Istanbul.
Turkey followed up by moving troops into Iraq, ostensibly to defend coalition forces training Iraqi Sunni and Kurdish fighters near Mosul, the self-declared Islamic State capital in Iraq.
Baghdad vehemently protested the Turkish presence, and Turkish flags were burned at protests across Iraq.
Washington had been slow to react as it ramped up in Incirlik. But Vice President Joe Biden eventually warned Turkish Prime Minister Ahmet Davutoglu that his government had to obtain permission from Iraq to deploy troops there.
On Monday, Davutoglu announced a "rearrangement" of some Turkish troops as a "necessary [step] from a military point of view."
Baghdad remained unsatisfied, however, and alleged that Turkey kept troops in the area. On Tuesday, Iraqi Prime Minister Haider Abadi demanded "a complete withdrawal from Iraqi territory and respect for its national sovereignty."