The Taliban have found success beyond their traditional strongholds in the rural south and are now dominating territory near crucial highways and cities that surround Kabul, the capital, in strategic provinces like Kapisa and Nangarhar.

Their advance has gone unreported because most American forces have left the field and officials in Kabul have largely refused to talk about it. The Afghan ministries have not released casualty statistics since an alarming rise in army and police deaths last year.

At a time when an election crisis is threatening the stability of the government, the Taliban’s increasingly aggressive campaign is threatening another crucial facet of the American withdrawal plan, full security by Afghan forces this year.

“They are running a series of tests right now at the military level, seeing how people respond,” one Western official said, describing a Taliban effort to gauge how quickly they could advance.