US President Donald Trump on Monday threatened to pull the Republican National Convention out of North Carolina unless the state's Democratic governor guarantees he will swiftly lift restrictions on large gatherings of people during the coronavirus pandemic.

Trump, who is set to be named the party's candidate for the November election, fired off a series of early morning tweets on Memorial Day, when the US honors it war dead, professing his "love" for the state but lamenting the fact that he might not be allowed to fill the Charlotte arena where the gathering is scheduled to be held between August 24-27.

"Unfortunately, Democrat Governor, @RoyCooperNC is still in Shutdown mood & unable to guarantee that by August we will be allowed full attendance in the Arena," the president wrote.

"In other words, we would be spending millions of dollars building the Arena to a very high standard without even knowing if the Democrat Governor would allow the Republican Party to fully occupy the space."

The conventions are traditionally moments of great pomp and pageant when parties officially announce their candidate for the election, and the events play an important role in the political life of the country.

"Plans are being made by many thousands of enthusiastic Republicans, and others, to head to beautiful North Carolina in August," Trump said.

"They must be immediately given an answer by the Governor as to whether or not the space will be allowed to be fully occupied. If not, we will be reluctantly forced to find, with all of the jobs and economic development it brings, another Republican National Convention site," he warned.

North Carolina entered the so-called Phase 2 of reopening from lockdown last week, allowing restaurants to open to 50 percent capacity but still banning large gatherings of people.

While the Democratic Party is considering holding its own convention -- also scheduled for August -- either online or in some other format as yet to be decided, Trump has rejected any such idea for his party, despite the risk of contagion.