US stock futures have fallen, commodities slipped and Treasuries edged higher after Donald Trump threw a last-minute spanner in to pandemic relief plans by threatening not to sign a long-awaited stimulus bill in to law.

S&P 500 futures were down half a per cent by mid-morning on Wednesday in Asia, and European and British equity futures fell by the same margin as the news offset hints of progress toward a British trade deal with Europe.

In a video posted on Twitter, Trump said the bill – the result of months of wrangling in Congress – was “a disgrace” with too much foreign spending, adding he wanted to increase “ridiculously low” $US600 cheques for individuals to $US2000.

“Really … you got to be kidding,” said Andrew Brenner, head of international fixed income at NatAlliance in a note emailed after Trump’s message.

“Personally we think the President will sign the bill at the last possible moment … but the true reality star will wait until the end,” he said. “Bond markets close 2pm Thursday while stocks close at 1pm – it may go down to the last moment.”

Ten-year US Treasury futures rose two ticks and the yield on US 10-year government bonds fell one basis point in Asia after Trump’s tweet.

It also soured sentiment which had caught a boost after ITV’s political editor said in a late-night tweet that separate sources had raised the possibility of Britain and the European Union striking a trade deal on Wednesday.

Holiday-thinned markets are also jittery about a highly contagious new coronavirus strain, which has given support to the safe-haven US dollar and yen and dragged on the prices of growth-sensitive commodities from oil to iron ore and copper.

France will reopen borders to Britain on Wednesday, but much of the world has sealed it off after a significantly more transmissible mutated coronavirus variant was discovered spreading swiftly across southern England.

Scientists say there is no evidence that vaccines currently being deployed in Britain will not protect against this variant, but the possibility has begun to haunt traders.

“The worry right now is whether the vaccine is less effective,” said Bank of Singapore currency analyst Moh Siong Sim.

“All these unknowns are keeping markets edgy, especially when they have made quite a bit for the year from the rally. People are keen to say let’s take some money off the table.”

The US dollar index rose 0.1 per cent as the greenback generally hung on to small but broad overnight gains.

The Australian dollar edged higher to $US0.7540 and most other majors were pretty close to flat, with the euro at $US1.2171 and sterling at $US1.3390.

Asian stocks steadied, with MSCI’s broadest index of Asia-Pacific shares outside Japan up 0.1 per cent after three days of declines. Japan’s Nikkei rose 0.1 per cent, although gains were concentrated in healthcare and technology stocks.

Both indexes are up more than 60 per cent from March lows.

Oil prices fell to test lows made during a sharp sell-off on Monday, with Brent crude futures last down 1.5 per cent at $US49.35 a barrel and US crude futures down 1.5 per cent at $US46.34.