Nato has asked Britain to consider sending more troops to Afghanistan, the BBC understands.
Currently there are about 500 British troops in the country, providing security in Kabul and training at the Afghan Officer Academy.
The BBC understands the request was made within the last few weeks.
Nato's secretary general Jens Stoltenberg is due to meet Theresa May at Downing Street on Wednesday and is expected to discuss the subject.
The request for more troops comes as the US considers increasing its military presence in the country.
The US military and state department are recommending sending at least 3,000 more US troops to Afghanistan to fight the Taliban, US media report.
There are 13,000 Nato troops currently in the country, 8,400 of them US.
US combat operations against the Taliban officially ended in 2014, but special forces have continued to provide support to Afghan troops.
The UK was involved in the conflict in Afghanistan against the Taliban and al-Qaeda for 13 years, from 2001 to 2014.
The last UK combat troops left Afghanistan in October 2014, but 450 remained in order to train, advise, and assist local Afghan forces.
In July another 50 were sent to aid counter-terrorism efforts and provide leadership training.
By Jonathan Beale, defence correspondent
The timing of the request may not be great for Theresa May ahead of the election. But it won't come as a huge surprise.
The US military have made no secret of their wish for more troops in Afghanistan. Barack Obama resisted the pressure. President Trump appears to be more willing to give his commanders a free rein.
If the US increases its military presence then it expects allies to do the same. It is, after all, a Nato-led mission, at least in name.
The MoD has received a formal request, but no numbers have been mentioned. One defence source expected that any increase would be small - between 10% and 20% of the 500 British troops already there.
Also expect caveats as to what they will do - and where they'll be deployed. No one in the MoD wants a repeat of Helmand.
Last month, the Taliban announced the start of their "spring offensive" a week after killing at least 135 Afghan soldiers in a military compound near the northern city of Mazar-e Sharif.
The group said it would use military and political tactics and that its main target would be foreign forces.
On Saturday Taliban militants seized a district just a few kilometres to the west of Kunduz in their continuing attempts to take the northern city. Thousands of families have been forced to leave their homes.
Latest reports suggest the Taliban now also control the main road to the east of the city which links the province to the north-east and is also the main supply route for the capital, Kabul.
A Ministry of Defence spokesman said: "The UK keeps its contribution in Afghanistan under regular review to ensure it remains suited for the needs of the mission."
A Nato official told the BBC US authorities had written to the military alliance to ask about the future of its presence in Afghanistan.
The official added: "Allied leaders will consider future contributions at our meeting in Brussels later this month, and the issue will be examined in further detail by defence ministers in June."
A Nato source said they did not expect the secretary general to mention specific numbers in his meeting with Mrs May on Wednesday.