US President Barack Obama's approval rating has climbed over Republican rival Mitt Romney

After the just-ended Democratic nominating convention, Obama now holds a 48-to-45-per-cent lead over Romney among registered voters in Gallup's tracking of presidential election preferences.

That compared to Obama's earlier 47-to-46-per-cent lead.

The Gallup organisation noted that the Republican had experienced 'no discernible bounce' during the Republican convention, which ended on August 27.

Gallup, one of the country's most respected polling firms, also found that Obama's approval rating had improved to 52 per cent, the highest the president has enjoyed since May 2011, after the killing of Osama bin Laden in Pakistan.

Gallup warned that it was possible that the 'upticks' were short-lived 'and that the race will devolve back to a parity' by next week.

But the polling firm added: 'If Obama builds on and sustains his higher job approval rating and lead over Romney, it could signal a possible resetting of the presidential race' before the first debate on October 3.

Opinion polls have shown Romney and Obama in a dead heat for weeks.