With India battling a surge in coronavirus cases, the UK has said it doesn't have spare vaccines to send there at the moment, but will support India in other ways.
Less than 10% of India's population has so far had a Covid vaccination, and vaccine shortages have been reported in some parts of the country.
What vaccines does India have?
The UK Health Secretary, Matt Hancock, has said that India's main vaccine maker, the Serum Institute of India (SII) is "making and producing more doses of vaccine than any other single organisation".
While that may be true, the SII has been struggling to meet domestic and global demand.
Although numbers have come down, India gave out more than 4.5m doses on one day in early April, and it's planning to expand its vaccine programme by opening it up to all adults from 1 May.
But the SII chief, Adar Poonawalla, has said there will be delays in vaccine deliveries of between five to six months.
The state of Maharashtra, which has the biggest number of cases in India, has said it will have to postpone the roll out to all adults because vaccines are not available.
The two main vaccine manufacturers, SII (which makes the local version of AstraZeneca's vaccine called Covishield) and Bharat Biotech (which makes Covaxin), can together produce around 90 million million doses a month.
They have been promised $400m and $210m each from the Indian government to boost production.
The Indian authorities also halted large exports of Covishield in March, although some smaller donations to other countries were allowed to go ahead, as well as some of the supplies promised to the global Covax vaccine-sharing scheme.
India has also allowed the import of foreign-made vaccines such as Pfizer, Moderna and Johnson & Johnson, although as yet, none of these vaccine makers has applied for an emergency use licence in India.
India's drug regulator recently approved the Russian vaccine Sputnik V for emergency use, with plans for it to be made in India.
This will be for both the Indian market and for export, although production has not yet started.
What help is the US offering India?
The US says it has identified about 60 million doses of AstraZeneca vaccine, which could be shared with other countries as and when they became available.
It's not year clear how much of this might go to India.
The AstraZeneca vaccine is not yet authorised for use in the US.
The US administration also says it is making available "specific raw materials" for the manufacture of Covishield vaccines in India (made under licence from AstraZeneca).
Earlier this year, President Biden invoked the US Defense Production Act (DPA), giving US vaccine makers priority access to specialised equipment, such as pumps and filtration units.
India's biggest vaccine maker, the Serum Institute of India, which produces Covishield, complained of shortages of specialised materials from the US.
The firm said it had faced difficulties importing cell culture media, single-use tubing and specialised chemicals from the US.
Dr Sarah Schiffling, an expert on vaccine supply chains at Liverpool's John Moores University, says the pharmaceutical supply chain is very complex and specialised.
"Even when [global] demand is very high, new suppliers can't spring up as quickly as they would in some other industries, or at least those new suppliers would not be trusted."
The US has also said it will fund a "substantial expansion" of manufacturing for another Indian firm, Biological E, which is making the Johnson & Johnson vaccine.
It will help the company ramp up production to at least one billion doses by the end of 2022.
The Serum Institute of India (SII) said in January it was planning to ramp up production of Covishield and the US-developed Novavax (not yet licensed for use in India) to 100 million doses a month by March.
But the plans have been pushed back to June.
Last year, the SII agreed to supply an initial 200 million doses to Covax - the WHO-backed vaccine sharing programme to ensure availability to low and middle income countries - 100 million each of the AstraZeneca and Novavax vaccines.
The SII was expected to deliver the first 100 million doses between February and May - but Indian government data shows that it has so far delivered only 30 million (this includes 10 million set aside for India itself under Covax)
The SII has also made bilateral commercial deals to supply more than 900 million doses of AstraZeneca vaccine, and 145 million doses of Novavax, according to UN data.
The global vaccine alliance Gavi, a partner in Covax, is reported to have said that the SII is legally obliged to provide vaccines for the scheme.
The company confirmed recently that it had been sent a legal notice by the UK-based AstraZeneca pharmaceutical company concerning its obligations to supply vaccines - although it did not go into details.