BANGKOK, April 15 (Reuters) - Thailand reported 1,543 new coronavirus cases on Thursday (Apr 15), the sharpest increase since the start of the pandemic and the fourth record rise this week, amid a third wave of infections in the Southeast Asian country.

While Thailand has up to now managed to keep case numbers relatively contained compared to many other countries, the new outbreak comes as many have travelled during the country's Songkran new year holidays and with vaccination rates still low.

When asked whether lockdowns would be imposed, health official Chawetsan Namwat said measures were being formulated based on case numbers in each area and would be proposed to the coronavirus taskforce for approval on Friday.

"We have to divide up the areas based on seriousness, which is different and so measures have to be suitable for each area," Chawetsan told a briefing.

Authorities have already urged people to limit travel and begin working from home.

Shopping malls will also close early at 9pm and the banking association said branches outside malls will close at 3.30pm there would be limit on customer numbers.

Thailand's last major lockdown was in late March last year, with a curfew imposed in April, before seeing months of relatively relaxed curbs as cases stayed mainly contained.

Of the new infections reported on Thursday, 409 were in capital Bangkok, the epicentre of the outbreak.

The new cases took the total number of infections to 37,453, with deaths remaining at 97.

The latest outbreak has already caused some strains on the healthcare system in Bangkok. All positive cases have to be admitted into care under Thai rules, with 8,973 patients currently being treated.

Chawetsan denied the rate of vaccination was slow but said the country had received only a limited supply. Government data showed about 2,000 additional doses administered by Thursday from a day earlier out a total of 581,308.

Thailand has vaccinated less than 0.4 per cent of its population, trailing neighbours like Malaysia with 1.5 per cent and 14.6 per cent in Singapore, according to a Reuters estimate.

The country has received two million doses of China's Sinovac shots and 117,300 AstraZeneca shots.

The government's main vaccination strategy relies on locally-produced AstraZeneca shots, which are expected to start being delivered in June, with the aim of inoculating half of the adult population by the end of the year.