President Joe Biden has come up well short on his goal of delivering 80 million doses of coronavirus vaccine to the rest of the world by the end of June as a host of logistical and regulatory hurdles slowed the pace of US vaccine diplomacy.

Although the Biden administration has announced that about 50 countries and entities will receive a share of the excess Covid-19 vaccine doses, the US has shipped fewer than 24 million doses to 10 recipient countries, according to an Associated Press tally.

The White House said more will be sent in the coming days and stressed that Mr Biden has done everything in his power to meet the commitment.

It is not for lack of doses. All the American shots are ready to ship, the White House said.

Rather, it is taking more time than anticipated to sort through a complex web of legal requirements, health codes, customs clearances, cold-storage chains, language barriers and delivery programmes. Complicating matters even further is that no two shipments are alike.

One country requires an act of its cabinet to approve the vaccine donation, others require inspectors to conduct their own safety checks on the US doses, and still others have yet to develop critical aspects of their vaccine distribution plans to ensure the doses can reach people's arms before they spoil.

A US official said that as of Wednesday, all intended recipient countries had received formal US offers of a specific number and type of vaccine, and all legal and logistical hurdles on the US side had been cleared.

The White House declined to specify which nations were grappling with which local hurdles, saying it is working with recipient nations on an individual basis to remove obstacles to delivery.

"What we've found to be the biggest challenge is not actually the supply - we have plenty of doses to share with the world - but this is a Herculean logistical challenge," White House press secretary Jen Psaki said last week.

It took months for the US to get its domestic vaccination programme running at full throttle, and officials noted that Mr Biden only shifted the focus of the nation's Covid-19 response towards the global vaccination campaign less than two months ago.

Mr Biden announced the 80 million target on May 17, saying, "This will be more vaccines than any country has actually shared to date - five times more than any other country - more than Russia and China."

Even while missing his goal, Mr Biden has made the US the largest global vaccine donor, delivering more doses than either Russia or China, who have at times sought to leverage their vaccines for geopolitical gain.

The 80 million doses are meant as a down payment on a far larger plan to purchase and donate 500 million vaccine doses for the world over the next year. That plan, relying on a purchase contract from Pfizer that will begin delivering doses in August, remains on track, officials said.