OTTAWA, Jan 28 (Reuters) - Canadian provinces protested on Thursday about a likely shortfall in deliveries of Pfizer Inc's COVID-19 vaccine, but the federal government insisted the U.S. drugmaker would live up to its commitments.

The exchanges underscore growing tensions over the slow rollout of Canada's vaccination program caused in part by Pfizer cutting its promised deliveries for January and February.

Ottawa, which had initially said Pfizer would deliver four million doses by the end of March, told the provinces on Thursday it would now be 3.5 million doses.

"The federal government is failing Canadians. This is a grim situation that seems to be getting worse every week," said Tyler Shandro, Alberta's health minister.

Ontario premier Doug Ford said Pfizer "has let us down tremendously" and called for "a loud voice to make sure we get our fair share of vaccines."

Federal officials told reporters the vaccine dose figures had been given to the provinces for planning purposes.

"(Pfizer) have assured us we will receive four million doses ... the rub is the numbers don't add up to four million at this time," said Major General Dany Fortin, who is helping lead the inoculation campaign.

The reason for the discrepancy is Pfizer's assumption that six doses can be extracted from each vial rather than the usual five, an alteration Canadian regulators may not approve.

"Pfizer is very much aware of that and will ... scale up their number (of deliveries) if that is what is required," Fortin said.

A special type of syringe is needed to retrieve that sixth dose, which would otherwise be wasted. Canada has ordered 40 million of them, with two million expected to arrive next week, federal officials said.

In a statement, Pfizer said it remained on track to meet first-quarter shipment objectives.

Canada is also due to receive two million doses of Moderna Inc's COVID-19 vaccine by March 31 and said the entire country should have been inoculated at some point in September.

Canada has recorded a total of 19,533 COVID-19 deaths and 761,226 infections so far. Health authorities in several provinces say hospitals could be overwhelmed by a second wave of the virus. (Reporting by David ljunggren in Ottawa and Allison Martell in Toronto; Editing by Mark Heinrich and Bill Berkrot)