Qayara Air Base - Iraqi and US-led coalition forces have killed or gravely wounded more than 2 000 Islamic State fighters in the battle for Mosul since October, the top US commander in Iraq said on Sunday.Lieutenant General Stephen Townsend told reporters there are still an estimated 3 000 to 5 000 ISIS fighters defending Mosul. He applauded the efforts of Iraqi security forces, who began their offensive on October 17 in what has been billed a decisive phase of the anti-ISIS fight."By our calculations, we think we have killed or badly wounded over 2 000," Townsend said at a joint news conference with US Defence Secretary Ash Carter at Qayara air base.Townsend disputed any suggestion that the Islamic State has managed to fight the Iraqi government forces to standstill in Mosul.After citing the estimated 2 000 ISIS casualties, he added, "I don't think that suggests anything about a stalemate. This is a major urban area. Any army on the planet, including the United States Army, would be challenged by this fight.""The Iraqi army has come back from near-defeat two years ago, and now they are attacking this major city 400km from Baghdad," Townsend said. "I don't think there is anything in there about a stalemate."Townsend said US intelligence estimated before the Mosul campaign kicked off in October that ISIS had 3 500 to 6 000 fighters in the city. He said the current estimate is 3 000 to 5 000.US officials have declined to say how many Iraqi government troops have been killed in the Mosul fight.Carter made an unannounced visit on Sunday to the Qayara base, flying in from Baghdad after meeting with Prime Minister Haider al-Abadi and sharing lunch with American troops. It was Carter's first visit to Qayara since it began operating as an Iraqi staging base in October.Carter toured the air base, greeting soldiers and offering holiday wishes. He assured them the Mosul campaign is on track.According to plan"Everything is going according to the plan of a year ago," Carter saidSunday's visit came as Iraqi security forces have been slowed in their nearly two-month-old offensive against ISIS, which has occupied Mosul for more than two years.In Bahrain on Saturday, Carter announced he is sending another 200 troops to Syria to train and advise local fighters combatting ISIS. There are already 300 US troops authorised for the Syria effort, and about 5 000 in Iraq.The recapture of Mosul, the country's second largest city, is crucial to the Iraqis' hopes of restoring their sovereignty, although political stability will likely remain a challenge afterwards.Carter told an international security conference in Bahrain that the battle for Mosul and for the Syrian city of Raqqa, the de facto capital of the extremists' self-described caliphate, would be crucial for defeating the group, which has claimed attacks worldwide."The seizure of these two cities is necessary to ensure the destruction of ISIL's parent tumour in Iraq and Syria - the primary objective of our military campaign - and put ISIL on an irreversible path to a lasting defeat," he said, using another acronym for ISIS.He did not predict how long it might take for Iraqi forces to prevail in Mosul, but he sounded a note of optimism."This is a complex mission that will take time to accomplish, but I am confident that ISIL's days in Mosul are numbered," he said in Bahrain.Iraqi forces have only captured a handful of eastern Mosul neighbourhoods since launching the offensive in mid-October. On Sunday they came under mortar fire as they worked to clear villages along the Tigris River to the south, part of operations to secure supply lines for a campaign that is likely to stretch into the coming year.Carter, whose tenure as defense secretary will end in January if his designated successor - retired Marine General James Mattis - is confirmed by the Senate as expected, also made the case for keeping US forces in Iraq even after the Islamic State group is dislodged from Mosul."Beyond security, there will still be towns to rebuild, services to re-establish, and communities to restore," he said in Bahrain. The extremists, he predicted, will attempt to survive by reinventing themselves "in some other shape or form" after they lose their grip on Iraq and Syria.Left unaddressed by Carter during his visit to Iraq was a possible change in course under president-elect Donald Trump when he takes office next month.