President Barack Obama has acknowledged that US intelligence underestimated the rise of Islamic State fighters in Iraq and Syria, where the head of an al-Qaida branch warned insurgents will attack the West in retaliation for US-led air strikes.US-led strikes have so far failed to halt an advance by fighters in northern Syria on Kobani, a Kurdish town on the border with Turkey where the past week’s battle caused the fastest refugee flight of Syria’s three-year civil war.His acknowledgment in an interview broadcast on Sunday that US intelligence had underestimated Islamic State offered an explanation for why Washington appeared to have been taken by surprise when the fighters surged through northern Iraq in June.The jihadists had gone underground when US forces quashed al-Qaida in Iraq with the aid of local tribes during the US war there that ended in 2011, Obama told CBS’s 60 Minutes.“But over the past couple of years, during the chaos of the Syrian civil war, where essentially you have huge swathes of the country that are completely ungoverned, they were able to reconstitute themselves and take advantage of that chaos.”Some of the US president’s opponents at home have seized on a remark he made in January using a sports metaphor to dismiss Sunni terrorists in Iraq and Syria. He compared them to a low-level school basketball team posing as professionals.“If a JV [junior varsity] team puts on Lakers uniforms, that doesn’t make them Kobe Bryant,” Obama told The New Yorker magazine in January.Al-Qaida’s branch in Syria, the Nusra Front and its Islamist rival, Islamic State, are coordinating their war strategies and may be moving towards some kind of alliance. A senior source told The Guardian that Nusra Front and Islamic State leaders are holding war-planning meetings, though no official pact has been signed.In addition, there seems to be a growing trend of Nusra Front members defecting to Islamic State. Last Friday, 73 members of the al-Qaida affiliate defected to Islamic State and more plan to do the same in coming days, a senior Nusra member told the British paper.The head of Nusra Front, which has been targeted by US strikes, said Islamists would carry out attacks on the West in retaliation for the campaign.“Muslims will not watch while their sons are bombed.Your leaders will not be the only ones who would pay the price of the war. You will pay the heaviest price,” Abu Mohammad al-Golani said in an audio message posted on pro-Nusra forums.Golani said his followers should not take advantage of the US strikes to hit out at Islamic State. The US strikes have created pressure on Nusra to reconcile with Islamic State, a move that would unite Syria’s most powerful Sunni forces and widen territory under their control.The group said that the US-led air strikes would fail. Meanwhile, Turkish tanks took up positions on the Syrian frontier, opposite a besieged border town where Islamic State shelling intensified and stray fire hit Turkish soil.At least 15 Turkish tanks were positioned at the frontier, some with guns pointed toward Syrian territory. More tanks and armored vehicles moved toward the border after shells landed in Turkey on Sunday and Monday.The United States has been bombing Islamic State and other groups in Syria for a week with the help of Arab allies, and hitting targets in neighboring Iraq since last month. European countries have joined the campaign in Iraq but not in Syria.Islamic State’s advance has not been halted in Syria, where it is fighting Kurdish forces near the border city of Kobani, where 140,000 refugees fled a week ago.Gunfire rang out from across the border and a plume of smoke rose over Kobani as periodic shelling by Islamic State fighters took place. Kurds watching the fighting from the Turkish side of the border said the Syrian Kurdish group, the YPG, was putting up a strong defense.“Many Islamic State fighters have been killed. They’re not taking the bodies with them,” said Ayhan, a Turkish Kurd who had spoken by phone with one of his friends fighting with the YPG.He said Kurdish forces had picked up eight Islamic State bodies.At Mursitpinar, the nearby border crossing, scores of young men were returning to Syria saying they would join the fight. More refugees were fleeing in the opposite direction.“Because of the bombs, everyone is running away. We heard people have been killed,” said Xelil, a 39-yearold engineer who fled Kobani on Monday. “The YPG have got small weapons, but Islamic State has big guns and tanks.”A local official inside Kobani said Islamic State continued to besiege the town from the east, west and south and that the fighters were 10 km. from the outskirts.“From the morning there have been bomb shellings into Kobani and... maybe about 20 rockets,” Idris Nassan, deputy foreign minister in a local Kurdish administration said by phone.He said the rockets had killed at least three people in the town.Turkey has not permitted its own Kurds to cross to join the battle: “If they’ve got Syrian identity or passports, they can go. But only Syrians, not Turks,” said one Turkish official at the border where security has been tightened.A NATO member with the most powerful army in the area, Turkey has so far kept out of the US-led coalition, angering many of its own Kurds who say the policy has abandoned their cousins in Syria to the wrath of Islamic State fighters.