WASHINGTON - The Obama administration is expressing alarm about an escalation of violence between Sudan and South Sudan and is calling for the leaders of the two nations to meet to resolve their differences.

Secretary of State Hillary Rodham Clinton said Tuesday that the leaders must reschedule a summit that had been scheduled for next week but was cancelled when Sudan's president said he would not attend. She said both sides have to end the violence but that the greater responsibility lies with Sudan, which has more advanced weapons.

Her comments, along with a statement of deep concern from the White House, came as Sudan's military bombed an oil field in the new nation of South Sudan a day after the two sides clashed in a disputed border town..