US President Joe Biden has reiterated his intention to stay in the presidential election race amid media reports that major Democratic donors are calling for him to step aside.

Concerns about Biden's age grew late last month, when the 81-year-old gave a shaky performance during his televised debate against his Republican opponent, former President Donald Trump.

Biden on Sunday addressed an audience at a church in Pennsylvania, one of the battleground states in the November election. He highlighted his intention to continue his election drive, saying his goal is to "unite America again."

But Biden remains under sustained pressure to quit the campaign, with five Democrats in the House of Representatives openly seeking his withdrawal.

ABC News says Biden's interview with the broadcaster on Friday and other events after his faltering debate performance "have not quelled fears of some major Democratic donors."

The Washington Post also reports that the "next few days will be pivotal in determining whether Biden can maintain the confidence of his party."

Lawmakers are returning for a congressional session on Monday after the Independence Day holiday. Biden is scheduled to hold a solo news conference on Thursday, the last day of a NATO summit due to open in Washington on Tuesday.