UN Secretary-general Ban Ki-moon on Thursday expressed his condolences to people severely affected by Hurricane Sandy and pledged assistance to help them recover.

The UN Headquarters in New York, located along the East River, were closed for three days due to the storm and suffered damage from high winds and flooding. It reopened on Thursday.

"Despite disruptions, all essential operations went ahead, including a meeting of the Security Council, and contacts with peacekeeping and other missions around the world were maintained without interruption," said a statement.

With superstorm Sandy dissipating, Obama and his Republican challenger Mitt Romney Thursday were back on the campaign trail, trying to make up for the lost time as they dashed towards the finish line.

Obama, who canceled campaign events for the first half of the week, held three events Thursday. Besides Wisconsin, he also visited Las Vegas, Nevada, and Denver, Colorado, trying to solidify his razor-thin lead in those swing states.

Romney, on the other hand, played defense in Virginia, where latest polls show his momentum has stalled. He held three events in Roanoke, Doswell and Virginia Beach, all in the Republican leaning southern part of the state.

The two candidates were making closing arguments before Americans head to polling stations across the country next Tuesday amid the question, "Where will the voters go to cast their ballots - particularly for parts of New York City and New Jersey where hundreds of thousands of people remained without power?"

Local election boards are assessing their polling sites to decide which ones would be up and running by Tuesday's election while utility companies hope to get the electricity back on time for Election Day.