The US and Britain have advised their citizens to leave Afghanistan immediately because of the worsening security situation in the country.
An advisory issued by the US embassy in Kabul on Saturday urged Americans "to leave Afghanistan immediately using available commercial flight options".
The embassy said its ability to assist US citizens in Afghanistan was "extremely limited even within Kabul" because of the security conditions and reduced staffing.
The US warning came a day after the UK's Foreign, Commonwealth and Development Office asked British citizens to leave Afghanistan.
"If you are still in Afghanistan, you are advised to leave now by commercial means because of the worsening security situation," it said.
The warnings came after the Taliban assassinated a senior government spokesman in Kabul on Friday and captured their first provincial capital since increasing attacks across the country in May.
Zaranj, the capital of south-western province of Nimroz, fell "without a fight", deputy provincial governor Roh Gul Khairzad told AFP.
A representative for the provincial police told Reuters the Taliban were able to capture the city because of a lack of reinforcements from the government.
The Afghan Defence Ministry said an air strike killed the Taliban's "shadow governor" for Nimroz, Abdul Khaliq, and 14 other militants at a gathering in Zaranj on Friday night.
The Taliban's nationwide offensive coincides with the withdrawal of US-led foreign forces after nearly two decades of conflict. The insurgents now control vast parts of rural Afghanistan and are challenging government forces in several cities, including Herat, near the western border with Iran, and Lashkar Gah and Kandahar in the south.
The militants carried out two attacks against officials in the capital past week to put pressure on the government to stop carrying out air strikes on its fighters.
Defence Minister Bismillah Mohammadi escaped an assassination attempt in a bomb-and-gun attack on Wednesday, but Dawa Khan Menapal, chief of of the government's media office, was killed by Taliban gunmen near a mosque in Kabul on Friday.
Taliban attacks on towns and cities are dragging the country into a “deadlier and more destructive” phase of urban warfare, the UN envoy to Afghanistan said on Friday.
Deborah Lyons told the Security Council that Afghanistan was likely to face more of the urban fighting, mass civilian casualties and pitched street battles that were reported in the siege of Sarajevo during the breakup of Yugoslavia in the 1990s, and in Syria's towns and cities over the past decade.
Russia's UN ambassador, Vassily Nebenzia, said the prospect of Afghanistan slipping into full-scale and protracted civil war was "a stark reality".
Britain's ambassador to the UN, Barbara Woodward, said the council "should leave the Taliban in no doubt there will be consequences for them if they continue to pursue this military offensive" and pledged that Britain would not recognise a Taliban government that came to power by force.
Britain's Foreign Office said its citizens should not rely on it to take them to safety, saying the assistance it could provide was "extremely limited".
"Terrorists are very likely to try to carry out attacks in Afghanistan. Specific methods of attack are evolving and increasing in sophistication," it said.