Oil prices rallied around 2.5 percent after eight members of the OPEC+ group of producers said Sunday they would extend supply cuts until the end of next month.

They had been delaying output hikes on worries about slowing demand in China and the United States.

While Asian markets gained, tracking a positive lead from Wall Street ahead of the weekend, European and US markets were mostly lower.

"Traders are gearing up for perhaps the most important week of the year," said Joshua Mahony, chief market analyst at Scope Markets.

Investors are looking for any hint of an advantage between the US presidential candidates as Democratic Vice President Kamala Harris and her Republican rival Donald Trump remain neck and neck in opinion polls ahead of Tuesday's poll.

The dollar retreated against its main rivals Monday as a fresh survey in Iowa, which Trump won in 2016 and 2020, showed Harris leading.

A victory for Trump is seen as being positive for the dollar and pushing up Treasury yields owing to his pledges to cut taxes and impose hefty tariffs on imports.

Elections for the Senate and House of Representatives are also being closely watched amid speculation the Republicans could take control of both.

"If the Republicans sweep all three, that will open the door to significant fiscal changes, which is negative for bondholders and could spell higher yields until the dust settles," said Peter Esho, founder of Esho Capital.

The election comes before the Federal Reserve is due to make its latest policy decision this week, with investors expecting 25-basis-point reduction after a bumper 50-point cut at its last gathering.

With the candidates running neck-in-neck, little can be said with certainty.

"In any case, volatility is on tap this week for all capital markets," said Briefing.com analyst Patrick O'Hare.

The vote is of particular interest to China, where officials in Beijing are meeting this week to hammer out an economic stimulus.

"We believe the US election results will have some impact on the size of Beijing's stimulus package," Nomura's chief China economist, Ting Lu, said in a research note.

Both candidates in the race have pledged to get tougher on Beijing, with Trump promising tariffs of 60 percent on all Chinese goods coming into the country.

Economists expect lawmakers to approve around one trillion yuan ($140 billion) in extra budget spending, mostly for indebted local governments, and a one-off one-trillion yuan payment for banks.

Hong Kong made gains and Shanghai was up more than one percent at the close. Tokyo was shut for a holiday.

In European trading, both Paris and Frankfurt were lower in afternoon deals.

London bucked the trend, gaining 0.4 percent, with the Bank of England widely expected to cut its main interest rate on Thursday after inflation dropped below its target rate.

On Wall Street, the S&P opened flat, but soon dipped into the red like the Dow and Nasdaq Composite.

Oil prices also firmed after Iran's supreme leader Ayatollah Ali Khamenei warned at the weekend that Israel and the United States "will definitely receive a tooth-breaking response" to Israeli attacks on October 26.

That strike was in response to an October 1 barrage of about 200 missiles against its rival.