WASHINGTON, June 28 (Reuters) - The Taliban has infiltrated an area of Afghanistan once seen by the United States and NATO as secure, boosting ranks and giving fighters strategic access to Kabul, a U.S. commander said on Thursday.

U.S. Army Col. Jonathan Ives, responsible for operations in five provinces in northeastern Afghanistan, said the Taliban had boosted recruiting in part of Kapisa, about 30 km (19 miles) north of the capital city.

"What we've seen is an escalation of force in there from about 50 to 200 -- about fourfold -- this year," Ives told reporters by videolink from Bagram Air Base near Kabul.

He said the area had been "somewhat ignored" by coalition and Afghan security forces.

"We thought that it was safe and secure in this province and so we considered it to be a non-threat area and so we didn't apply or maintain a security force," Ives said.

"(The Taliban) did fill that vacuum in this case."

Violence has risen over the past 18 months, the bloodiest period since the Taliban government was ousted after a U.S.-led invasion in 2001.

Senior U.S. defense and military officials have downplayed Taliban activities this spring, saying the expected offensive after winter snows melted had not materialized.

But U.S. and NATO officials have refused to release data on the number of Taliban attacks this year.

U.S. Army Brig. Gen. Joseph Votel, NATO's deputy commander in eastern Afghanistan, this week said attacks had increased, including the use of improvised explosive devices (IEDs) and suicide bombings.

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