US President Barack Obama widened his lead over Republican challenger Mitt Romney to 7 percentage points in a Reuters/Ipsos poll of likely voters, the latest survey to show the Democrat ahead in the run-up to the November 6 election.

The daily online poll asked 990 likely voters over the previous four days which candidate they would pick if the vote took place today, with 48 per cent choosing Obama and 41 per cent picking Romney.

The gap has been widening since Obama grabbed the lead in the rolling poll on September 7 when he scooped up 46 per cent of likely voters to Romney's 44 per cent after the Democratic convention.

"What that really means is that Obama is in good shape," said Ipsos pollster Julia Clark, attributing some of Obama's uptick to the slowly improving sentiment toward the direction of the country shown in Wednesday's (local time) telephone poll.

Among all 1,231 registered voters surveyed online, Obama led with 45 per cent to Romney's 39 per cent.

Thursday's online poll also found far more registered voters preferred the incumbent's policies and approach on taxes (41 per cent picked Obama, 30 per cent Romney), healthcare (44 per cent Obama, 28 per cent Romney) and Social Security (39 per cent Obama, 27 per cent Romney).

Asked which of the candidates had a better plan, policy or approach to the war on terrorism, more registered voters again favoured Obama: 39 per cent to Romney's 25 per cent.

Foreign policy moved to the center of the campaign this week after four Americans, including an ambassador, were killed in Libya as protests raged in Benghazi and Cairo in neighbouring Egypt against an anti-Islam film made in the United States.

The two candidates ranked closely on the US economy: 36 per cent said Obama's approach was better, versus 35 per cent for Romney.

Obama held a slight lead of 38 per cent to Romney's 35 per cent on jobs and unemployment, despite poor unemployment figures last Friday.

But 35 per cent of registered voters found Romney's policies and plans on the federal deficit were better than Obama's. Thursday's poll showed Obama with 31 per cent.

Independents - a key voting bloc - favoured neither of the candidate's policies on many issues.

"Neither candidate has established credibility on these issues," Clark said. "Neither candidate is really doing it for these independents."

Asked for whom they would vote for in Thursday's poll, the small pool of independents preferred Romney with 35 per cent to Obama's 26 per cent, which Clark attributed to their high focus on economic issues.

The precision of Reuters/Ipsos rolling daily online polls is measured using a credibility interval.

In this case, the poll has a credibility interval of plus or minus 3.2 percentage points for all respondents.

- Reuters