The government requires all EU nationals living in Britain before December 31, 2020, to register for a new immigration status managed and verified entirely online.
Wedged between Ukraine and EU member Romania, Moldova has long been divided over closer ties with Brussels or maintaining Soviet-era relations with Moscow
This week, Portugal, Spain and Malta all abruptly increased restrictions for travellers from the UK, although the three said they would accept fully vaccinated Britons.
The EU levy, due to be announced by the European Commission on July 14, "threatens the work being undertaken via the OECD/G20 process," said the US document.
The coronavirus pandemic has masked the economic dislocation caused by the referendum verdict, which backed severing five decades of integration with the European mainland.
Negotiated in marathon talks by EU and US officials, the truce will be formalised in Biden's summit with European leaders Charles Michel and Ursula von der Leyen.
"But there is a lack of reciprocity and there is a lack of fairness and that's why we tried last year to accelerate the negotiations in relationship with this investment agreement.