The Islamic State's black flag flew Monday near one of the gateways to the strategic Syrian town of Kobane, suggesting that the militants have broken through more defensive lines after weeks of intense fighting within sight of the Turkish border.

Capturing Kobane would give the Islamic State control over a longer stretch of the Turkish-Syrian border and open potential new smuggling lines for fighters and supplies.

NATO member Turkey has authorized its military to cross the border to confront the militant group, but Turkish commanders have held back their tanks and troops to aid Syrian Kurdish forces trying to hold Kobane.

Kobane's fall also would highlight the limitations of U.S.-led airstrikes to blunt ground offensives by well-armed Islamic State units. In Iraq, the group's fighters have gained territory against the military and allied militiamen despite facing intensified air attacks.

The U.S. military said its forces and those of partner nations conducted three strikes against Islamic State targets in Syria, including south of Kobane, on Sunday and Monday using fighter and remotely piloted aircraft. Separately, U.S. military forces used remotely piloted aircraft and helicopters to carry out three strikes in Iraq.

The deployment of U.S. attack helicopters to the battlefield in Iraq suggests that Iraqi troops need closer air support as they fight Islamic State militants, who have inched nearer to Baghdad in recent weeks.

The aircraft are known for their accuracy and can fire missiles from several miles away. They also can help hit mobile targets such as armed trucks or ground troops, said Chris Harmer, a retired Navy helicopter pilot who is now an analyst with the Institute for the Study of War.

"The more lively the fight is, the more likely you are to need rotary-wing aircraft to assist forces on the ground," Harmer said.

But because helicopters fly at low altitudes, they are also more vulnerable to rocket-propelled grenades and other weapons.

The Islamic State has demonstrated its ability to take down aircraft in both Iraq and Syria. Last week, members of the jihadist group shot down an Iraqi helicopter near the Baiji oil refinery in northern Iraq. The group also overran Iraqi army bases in the west, prompting intensified strikes from coalition warplanes over the weekend.

Outside Kobane, meanwhile, the Islamic State has steadily pushed back Syrian Kurdish forces since mid-September as more than 160,000 refugees have fled to Turkey or nearby areas.

Some of the heaviest fighting left more than 45 people dead Sunday, including 10 Islamic State fighters killed by a Kurdish female suicide bomber, said the Britain-based Syrian Observatory for Human Rights. A Kurdish militia group placed the overall death toll at close to 90.

But the Islamic State appeared to have pushed to the edge of Kobane, also known as Ayn al-Arab in Arabic. The Islamic State's black banner with Arabic writing was raised over a building at the eastern entrance to the town, according to the Associated Press and other news agencies watching from across the Turkish border.

Clashes flared on several fronts, but it was unclear whether Islamic State fighters were able to penetrate deeper into the town. There are few retreat options for Syrian Kurdish fighters, who may attempt to seek shelter in Turkey if the Islamic State advances — possibly drawing that country further into the conflict.

Turkish Defense Minister Ismet Yilmaz told reporters that NATO has drawn up a strategy to defend Turkey if it is attacked along its border with Syria. In the event of an attack, he said, "NATO's joint defense mechanisms will be activated," according to the AP.

During a visit to Poland, NATO's secretary general, Jens Stoltenberg, promised much the same, saying the alliance will stand by Turkey if it faces attacks from the Islamic State.

"Turkey should know that NATO will be there if there is any spillover, any attacks, on Turkey as a consequence of the violence we see in Syria," Stoltenberg said at a news conference. - Brian Murphy and Erin Cunningham