The United States called on Tuesday for a halt to deeply distressing violence between Sudan and South Sudan, and urged both countries' leaders to go ahead with a summit next week despite fears it may be scrapped.

I've been following this closely and it's been a painful problem to see the deterioration into conflict again, U.S. Secretary of State Hillary Clinton told reporters as some of the most serious fighting flared between the two countries since the South declared independence from Sudan in July.

The violence has ended a recent rapprochement between the neighbours, which remain divided on a number of issues including the division of oil revenues. Each country has also accused the other of supporting rebels on either side of the border.

Clinton said Washington, which helped to broker the 2005 peace deal which ended Sudan's long civil war, held Khartoum primarily responsible for the latest hostilities, in which the two neighbours have traded accusations of attacks on either side of their contested border.

Sudan state media reported on Monday that Sudan President Omar Hassan al-Bashir had decided to postpone a summit with his southern counterpart scheduled for April 3.

We want to see that summit held, and we want to see both sides work together to end the violence, Clinton said, calling it deeply distressing.

We think that the weight of responsibility rests with Khartoum because the use of heavy weaponry, bombing runs by planes and the like, are certainly evidence of disproportionate force on the part of the government in Khartoum, she said.

At the same time we want to see South Sudan and their allies, or their partners, across into Sudan similarly participate in ending the violence and working to resolve the outstanding issues.

The White House on Tuesday also called for restraint amid what it called on alarming upsurge in violence and urged the two sides to negotiate disagreements in disputed border regions including Abyei, Blue Nile and Southern Kordofan, where hundreds of thousands of people have fled the fighting.

South Sudan and Sudan have differing accounts of the latest violence, and events along the 1,800-km (1,100-mile) border area are hard to verify as much of the territory is disputed and barred to journalists.

Clinton said the two sides -- bound inextricably by their intertwined oil industries -- should focus on finding a solution to their deteriorating economies.

There is a win-win outcome here. South Sudan has oil. Sudan has the infrastructure and the transportation networks to get the oil to market, Clinton said.

There are decades of grievances that have to be overcome in order to work through these very challenging issues. But it is incumbent upon the leaders of both countries to attempt to do so.

(Reporting By Jeff Mason and Andrew Quinn; Editing by David Brunnstrom)