Myanmar's president has signed off a law requiring some mothers to space their children three years apart despite objections by a visiting US diplomat and rights activists.

The bill, which was drafted under pressure from hard-line Buddhist monks, was passed by parliamentarians last month and approved by President Thein Sein, state media reported on Saturday.

The announcement came a day after US Deputy Secretary of State Anthony Blinken warned during a face-to-face meeting with Sein of the potential consequences of the law, the Associated Press news agency reported.

Both the US and rights activists worry it could be used not only to repress women, but also religious and ethnic minorities.

Blinken said he expressed "deep concern" about the law and three others in the assembly aimed at protecting race and religion.

"The legislation contains provisions that can be enforced in a manner that would undermine reproductive rights, women's rights and religious freedom," Blinken said.

"We shared the concerns that these bills can exacerbate ethic and religious divisions and undermine the country's efforts to promote tolerance and diversity."

"It's very disappointing," Khin Lay, a women's rights activist, said of the president's decision to sign off on the law.

"If the government wants to protect women, they should strengthen laws already in place to do that."

Fears of discrimination

The government says the law is aimed at bringing down maternal and infant mortality rates, activists argue that it steps on women's reproductive rights and can be used to suppress the growth of marginalised groups.

As predominantly Buddhist Myanmar started moving from dictatorship to democracy four years ago, newfound freedoms of expression lifted the lid on deep-seeded hatred for minority Muslims including Rohingya Muslims now arriving on Southeast Asian shores in crowded boats.

Many are fleeing persecution and violence that has left up to 280 people dead and forced another 140,000 from their homes in western Rakhine state. They are living under apartheid-like conditions in dusty, crowded camps, with little access to education or adequate medical care. They also have little freedom of movement, having to pay hefty bribes if they want to pass police barricades, even for emergencies.

The population law, which carries no punitive measures, gives regional authorities the power to implement birth-spacing guidelines in areas with high rates of population growth.

Hard-line Buddhists have repeatedly warned that Muslims, with their high birthrates, could take over the country of 50 million even though they currently represent less than 10 percent of the population.