(Adds comments from NATO, Taliban, police)
By Ahmad Sultan
LOGAR, Afghanistan, Aug 19 (Reuters) - As many as 700 heavily armed Taliban insurgents are battling Afghan security forces in Logar, a key province near the capital Kabul, local officials said on Tuesday, in a test of the Afghan military's strength as foreign forces pull out of the country.
Militants have this summer mounted increasingly intensive assaults across several provinces, often involving hundreds of fighters, as the country braces to stand on it own feet militarily for the first time in nearly 13 years.
"There are some 700 of them and they are fighting Afghan forces for territorial control and they have also brought with them makeshift mobile (health) clinics," Niaz Mohammad Amiri, the provincial governor of Logar province, told Reuters by telephone.
The Taliban have dug-in in Logar, which lies about an hour's drive south of Kabul, and nearby Wardak province to the west, in recent years. They have used the provinces - gateways to the capital - as launchpads for hit-and-run attacks and suicide bombings on Kabul.
The main roads into the capital are all tightly controlled, but the militants have still been able to breach the checkpoints and staged dozens of attacks, killing scores of civilians and soldiers in the city of about five million this year.
Abdul Hakim Esaaqzai, the police chief of Logar province, said the insurgents, armed with heavy machine guns, were fighting Afghan forces from residential areas in Charkh district.
"We are being extra careful not to cause any civilian casualties. We have enough forces to deal with it," Esaaqzai said.
Zabihullah Mujahid, a Taliban spokesman, said the militants were battling Afghan forces from all sides to overrun the district. "The area is under siege and we have already taken over many security outposts and killed many Afghan forces," he told Reuters by telephone.
The fighting in Logar is a grim reminder of the insecurity plaguing Afghanistan as the foreign combat troops wind down their military operations ahead a deadline to leave the country by the end of the year.
Afghanistan's security arrangements beyond 2014 are unclear, as Kabul and Washington have yet to sign a bilateral security agreement designed to keep a small force of American soldiers in Afghanistan next year and into 2016.
Afghan President Hamid Karzai has refused to sign the agreement, although the two men vying to replace him have vowed to implement it immediately upon taking office. However, four months after going to the polls, a winner in the presidential poll is still not clear.
The International Crisis Group said in a report earlier this year that the number of Islamist insurgent attacks had increased by 15-20 percent in 2013 from a year earlier.
The insurgents' success, however, has been limited. They have yet to capture an entire province, and the government says strategic assets remain broadly under its control.
Nevertheless, the mounting intensity of the Taliban's assaults poses an increasingly serious challenge to local security forces that have long relied on NATO support from the air.
With less and less U.S. air cover, Taliban fighters have changed tactics and now attack Afghan military posts in larger numbers with the aim of taking and holding ground, a shift from the hit-and-run strikes with posses of gunmen, explosives and suicide bombers.
"The Afghan National Security Forces (ANSF) have the lead for security of the nation. They are trained, capable, and prepared to face any challenge," said Major Paul Greenberg, a public affairs officer at ISAF Joint Command.
"ISAF will continue to back up the ANSF and provide support, to include aviation assets upon request of the ANSF," he added.
In this year's summer offensive, the Taliban appears to have mostly focused on gaining ground in strategic parts of the country, like border crossings or highways that facilitate the export of opium, the financial lifeblood of their insurgency.
Afghan officials say uncertainty in Kabul over the outcome of this year's presidential election to choose Karzai's successor has added to the vulnerability of the security forces.
Two months have passed since the run-off round of the election was held, but a winner has yet to emerge due to accusations of mass fraud and rivals Abdullah Abdullah and Ashraf Ghani have both claimed victory. (Additional reporting by Krista Mahr in Kabul; Writing by Hamid Shalizi; Editing by Jeremy Laurence)