BRASILIA, Jan 19 (Reuters) - Brazil is battling bureaucracy in China to free up exports of active ingredients for vaccines developed by AstraZeneca and Sinovac Biotech, three people familiar with talks told Reuters, without which an immunization push could soon slow to a trickle.

More Brazilian states gave their first COVID-19 vaccinations on Tuesday, as the government distributed some 6 million ready doses of the vaccine from China's Sinovac after its approval on Sunday for emergency use.

However the sources, who spoke anonymously due to diplomatic sensitivities, said red tape in China was holding back supplies needed for Brazil to finish and distribute millions more doses from its own biomedical facilities.

"It's a new situation, and there's a bureaucratic problem. The Chinese are still defining procedures, which takes time," one source said. "There's also a relative scarcity of supplies."

The person said Brazil was not the only country struggling with export hurdles. "It's not targeted at us."

Right-wing Brazilian President Jair Bolsonaro has repeatedly antagonized China. Recently, he disparaged the Sinovac shot based on its "origins."

Brazil's federally funded Fiocruz biomedical center said it would not be able to deliver finished doses of the AstraZeneca shot until March as it waits for the first shipment of active ingredients from China. The institute had been aiming for 1 million doses by mid-February.

A British government minister on Monday flagged concerns about a "lumpy" manufacturing process slowing the rollout of vaccines from AstraZeneca and Pfizer in the United Kingdom.

AstraZeneca has arranged for substantial manufacturing of its vaccine's active ingredients in China. Last month, Brazilian health inspectors visited and approved the facilities of Chinese firm WuXi Biologics to export the ingredients of the AstraZeneca shot for finishing in Brazil.

However, the first shipment to Fiocruz in Rio de Janeiro has been delayed repeatedly, leaving the facility there idle. Brazil's government is scrambling to import ready doses of the AstraZeneca vaccine from India, but has faced delays there too.

In Sao Paulo, the state-funded Butantan Institute has imported enough active ingredients for the Sinovac vaccine to fill and finish nearly 5 million doses, on top of the 6 million finished doses already imported and distributed nationally.

The next shipment of ingredients has been delayed and Butantan officials warned on Monday that if it does not arrive by the end of the month, the institute will be unable to hit its target of 46 million doses delivered by April.

"The Chinese side is doing its homework," said a second source, who is familiar with the Chinese government's thinking. "But the bureaucracy is very vigorous."

A third source said talks were advanced and the shipments should "soon" be cleared for export. (Reporting by Lisandra Paraguassu and Jake Spring Additional reporting by Pedro Fonseca Editing by Brad Haynes, Bernadette Baum and David Gregorio)