U.S. Senate delegation led by top Republican John McCain flew over Pakistan’s tribal areas on Monday after meeting Prime Minister Nawaz Sharif to discuss regional security, as Washington gears up to send more troops to neighbouring Afghanistan.

The visit by members of the influential Senate Armed Services Committee came days after Islamabad slammed Washington’s decision to sanction a Kashmiri militant leader. The relationship between the U.S. and Pakistan has been strained at times with some in Washington believing Islamabad has not done enough to bring its influence to bear to persuade the Afghan Taliban to renounce violence.

Mr. McCain said “close cooperation between the U.S. and Pakistan was essential for securing peace and stability in the region” at the meeting with Mr. Sharif and other top Pakistani officials in Islamabad, according to a statement from the PMO.

‘Essential partnership’

He also “attached importance to (U.S.) relations with Pakistan, which remained a close friend and ally”, the statement continued. Mr. Sharif also called the Pakistan-U.S. partnership “essential”, and reiterated Pakistan’s commitment to an Afghan-led peace process in Afghanistan, with strong relations between Washington, Kabul and Islamabad a “prerequisite”, according to the statement.

Later on Monday, the Senators flew with the military over South Waziristan in the Federally Administered Tribal Areas to see progress on infrastructure built there with U.S. assistance, including roads and border outposts, and dam and irrigation projects.

A statement from the military said they were also briefed on border security, including Pakistan’s bid to fence the frontier with Afghanistan.

Pakistan has received billions in U.S. aid since the invasion of Afghanistan in 2001. Washington is actively considering sending more troops to the war-torn country to help struggling Afghan forces beat back the resurgent Taliban, with American commanders requesting thousands of extra boots on the ground.

The Senate Armed Services Committee has legislative oversight of US military affairs. Other members of the delegation, which arrived Sunday and will leave Monday, included Senator Lindsey Graham, Senator Elizabeth Warren, Senator David Perdue and Senator Sheldon Whitehouse.

On Sunday they met with Pakistan’s top foreign affairs official and the chief of its powerful military.