* Outbreak could be worst one yet - health minister
* Tens of thousands of tests to be undertaken
* Virus reemerges as Vietnam holds key Communist Party congress
* 'We have to stay calm', PM tells emergency meeting (Recasts with news cases, emergency meeting, adds details and quotes throughout)
By Phuong Nguyen and James Pearson
HANOI, Jan 28 (Reuters) - Vietnam confirmed its first outbreak of the coronavirus in nearly two months on Thursday and warned it could be more serious than its last wave of infections, as authorities rushed to contain the spread and test tens of thousands of people.
The health ministry earlier on Thursday reported two locally transmitted cases, the country's first in 55 days, one of which was exposed to an individual who had tested positive in Japan for the more contagious B.1.1.7 UK variant.
The 83 infections announced on Thursday, were a new record daily for Vietnam which has effectively closed its borders and avoided the larger epidemics seen by its neighbours. Previously the country had recorded just over 1,550 cases and 35 deaths since the virus was first detected.
The outbreak, in Hai Duong and Quang Ninh provinces near the capital Hanoi, is the biggest in Vietnam since a cluster emerged in the central city of Danang in July and quickly spread to most regions of the country, leading to its first coronavirus fatalities.
"We think the outbreak in Hai Duong will be worse than the one in Danang," Health Minister Nguyen Thanh Long said at an urgent meeting on the sidelines of the Communist Party's five-yearly Congress.
"We will take tens of thousands of samples for testing today," he told the meeting, an audio recording of which was reviewed by Reuters.
Long said Vietnam had so far not seen so many tests come back positive: a total of 72 out of 138 people to have come into contact with one of the two new cases reported on Thursday morning tested positive for the virus, Long said.
The outbreak will be a jolt for Vietnam, which, owing to strict quarantine, testing and tracing measures, has kept the virus in check, earning it a top three spot in a survey of how well countries have handled the pandemic.
A village and an electronics factory was locked down on Thursday, with 2,340 workers quarantined in Hai Duong and huge contact tracing efforts launched.
Containment efforts could be complicated by an upcoming Lunar New Year holiday period, when big gatherings indoors are typical.
The outbreak also comes at an awkward time for Vietnam's ruling Communist Party, as it holds its most important gathering, during which a new national leadership will be decided.
"We've encountered such problem many times so we have to stay calm," Prime Minister Nguyen Xuan Phuc said at Thursday's emergency meeting, calling for strong measures but not to cause panic.
A first wave of infections was crushed in April, and the country went nearly 100 days without local transmission until the Danang outbreak. (Reporting by Phuong Nguyen, James Pearson and Khanh Vu; Writing by Martin Petty; Editing by Kenneth Maxwell)