By the BBC's Ishbel Matheson Nairobi

The United States Government has expressed deep concern about an upsurge in fighting in southern Sudan.

Washington called on the government and the rebel Sudan People's Liberation Army to concentrate on peace and end their military action.

Thousands of people are reported to have been displaced in clashes in the oil rich areas of Upper Nile and Bar el Ghazal states.

Over the past year, the US has stepped up its involvement in Sudan, and brokered a ceasefire in the remote Nuba Mountains area.

But the main battleground is the land around the oil fields.

Whoever controls Sudan's richest resource could ultimately win the long-running civil war.

Oil war

Government forces withdrawn from the Nuba Mountains are reported to have been redeployed around the oil fields of western Upper Nile.

Oil has helped Khartoum fund its war effort

They are apparently facing a stronger, more unified rebel force.

Traditionally, both sides scrambled to capture territory before the rains arrive.

In northern Bahr el Ghazal, a government offensive is said to have driven thousand of people from their homes.

In the US-brokered deal, both sides agreed civilians should not be targeted in this war.

Aid workers say this current round of fighting flies in the face of that agreement.