US Vice-President Dick Cheney has held talks with President Pervez Musharraf on a surprise visit to Pakistan.
Reports said Mr Cheney (left) was carrying a 'tough' message
Mr Cheney urged Pakistan to do more to combat the Taleban near the Afghan border, but also praised its role in the "war on terror", officials said.
Pakistan is a key US ally in the region but Washington has been alarmed by the Taleban's growing strength in its tribal areas bordering Afghanistan.
Mr Cheney spent only a few hours in Islamabad before flying on to Kabul.
His trip follows a report in the New York Times saying that US President George W Bush wants to send a tough message to Pakistan that Washington could cut its aid to the country unless President Musharraf goes after militants more aggressively.
It also coincides with one to Islamabad by UK Foreign Secretary Margaret Beckett.
The BBC's Barbara Plett in Islamabad says the US and Britain share concerns about an expected increase in Taleban fighting and the support the insurgents get from Pakistani tribes along the border with Afghanistan.
Pakistan has thousands of troops near the border
She says the Americans have made quite pointed statements in recent months about the need for Pakistan to eliminate Taleban safe havens.
There was no publicity around Dick Cheney's visit to the region.
He went straight into talks with President Musharraf on his arrival in Islamabad and made no public comment after his meeting.
Pakistani officials, however, said Mr Cheney had conveyed US "apprehensions" about al-Qaeda regrouping in the tribal areas near Afghanistan.
And he communicated "serious US concerns" about reports that a surge in Taleban attacks against Nato forces in Afghanistan was imminent, the officials said.
But he also praised Pakistan for its "pivotal role" in fighting the militants.
Mrs Beckett also met President Musharraf during her talks in Islamabad.
She briefed him about an expected announcement that more UK troops are being sent to Afghanistan.
She said Britain recognised the steps Pakistan had taken to prevent Taleban infiltration across the border, but that no-one wanted to see terrorist sanctuaries in Pakistan.
Pakistani officials said border security was a joint responsibility and that Nato and Afghan forces on the other side must do more.
Earlier this month, US said it would not neglect Afghanistan and allow extremists to take over the country.
There are 27,000 US troops in Afghanistan, the number highest since the invasion of 2001, to combat an expected spring offensive by the Taleban.
Afghanistan and Pakistan share a 1,400 mile (2,250km) border.
Many Taleban fighters operate from bases on the Pakistani side of the border and Nato and the Afghan government say Islamabad must do more to curb violence.
Correspondents say that Western officials acknowledge President Musharraf's difficulties, but they are afraid that the Taleban are using Pakistan to prepare for a spring offensive.
President Musharraf's government has also come under fire for pacts with tribal militants in the North and South Waziristan border areas. Critics say the deals give Taleban fighters based there freedom to go where they please.