KABUL  – Two American troops died in fighting in Afghanistan on Thursday, while NATO and local officials said coalition and Afghan forces had killed at least 37 insurgents in a series of ground and air engagements.

U.S. Defense Secretary Robert Gates, meanwhile, arrived in Afghanistan's capital for meetings with President Hamid Karzai and top NATO commander Gen. David Petraeus.

Gates flew Thursday morning to Kabul from Baghdad, where he participated in ceremonies marking the formal close of the U.S. combat mission in Iraq. The Pentagon chief also plans to visit U.S. troops in Afghanistan.

NATO said one service member was killed in the country's east, the other in the south regions where fighting between the coalition and Taliban insurgents has been at its most intense. No other details were given in keeping with standard NATO procedure.

The deaths bring to three the number of U.S. service members killed in September and follows a spike in casualties during the last two weeks of August that saw the monthly total rise to 55. The August figure was still below the back-to-back monthly records of 66 in July and 60 in June, although total U.S. combat deaths in January-August of this year  316  last month exceeded the previous annual record figure of 304 for the whole of 2009.

NATO said coalition forces beat back an attack on a combat outpost in Paktika province's Barmal district along the mountainous border with Pakistan, killing at least 20 insurgents.

Defenders first returned fire with mortars and small arms before calling in an air assault, the alliance said in a news release, adding that no NATO or Afghan government forces were killed.