The United States' top field commander, General David Petraeus, has warned of a tough mission ahead just one day after arriving to take command of the 150,000-strong Nato-led foreign force in Afghanistan.

Petraeus told hundreds of guests at a US embassy party held to mark US Independence Day that it was essential to show unity of purpose to solve Afghanistan's problems.

Petraeus is in-charge with not only winning the war against a growing Taliban insurgency, but also with starting a withdrawal of US forces from July next year.

He landed in Kabul after his appointment was confirmed by the US Senate and the US House of Representatives approved $33 billion in funding for a troop surge he hopes will turn the tide of the war.

Nearly 1,900 foreign troops have died in Afghanistan since the Taliban were overthrown in 2001 - including more than 100 last month, the bloodiest since the war began.