KABUL, Afghanistan — The Taliban have offered a brief period of reducing violence in Afghanistan during ongoing negotiations with United States diplomats, three officials familiar with the talks said on Thursday, a concession seen as important to finalizing a preliminary peace deal between the insurgents and the United States to end their 18-year war.

If the American side accepts the offer, it could amount to the most significant development in the yearlong negotiations since talks resumed after President Trump had scuttled the peace process on the eve of a deal in September.

Though the pledge to reduce violence falls short of the overarching long-term cease-fire sought by the Afghan government, Western diplomats had said getting the Taliban to agree to more than a modest reduction in attacks would be difficult before the withdrawal of foreign forces gets underway.

Details of the offer, confirmed by Western and Taliban officials familiar with the negotiations, were unclear, though the Taliban have said in the past that a reduction in violence would mean scaling back attacks on major cities and highways.