At least five children have drowned after a boat carrying Rohingya refugees fleeing violence in Myanmar sank early on Wednesday, Bangladeshi border guards have said.

Authorities, according to the Associated Press, said the boat sank at the mouth of the Naf river, which separates Bangladesh from Myanmar's violence-wracked Rakhine state, raising fears there could be many more casualties.

"So far, the bodies of five male and female children have been found at different locations," Border Guard Bangladesh Officer Aloysius Sangma told AFP news agency which said three to four boats had sunk, citing border officials.

Last week, Bangladeshi border guards recovered two dozen bodies from the shore as an upsurge in violence in Rakhine on August 25 increased efforts to cross the border.

Authorities in Myanmar say close to 100 people have been killed in the violence, but advocates for the Rohingya told Al Jazeera that at least 800 of the Muslim minority, including dozens of women and children, were dead.

Al Jazeera has been unable to independently verify the figures.

According to a UN estimate, more than 146,000 refugees have crossed the border into Bangladesh, and UNICEF, the UN children's agency, said 80 percent of those are women and children.

Myanmar's national security adviser, however, disputed those claims, saying the number of people crossing the border was "closer to 20,000 than the hundreds of thousands".

READ MORE: Who are the Rohingya Muslims?

On Tuesday, Reuters news agency reported that hundreds of exhausted Rohingya arrived near the Bangladeshi border village of Shamlapur by boat.

The new arrivals - many of them sick or wounded - have strained the resources of aid agencies and communities already helping hundreds of thousands of refugees from previous spasms of violence.

A UN source working in Bangladesh's Cox's Bazar district said many of those arriving had no shelter, and aid agencies were racing to provide clean water and sanitation.

Against this backdrop of a growing humanitarian crisis, Turkey has pledged to send 1,000 tonnes of aid to Rakhine after President Recep Tayyip Erdoganspoke to Aung San Suu Kyi, Myanmar's civilian leader, according to government spokesperson.

In Pictures: The perilous journey of Rohingya refugees

Erdogan condemned the escalating human rights violations against the Rohingya during a phone call with Aung San Suu Kyi earlier on Tuesday, Turkish presidential sources said.

"People have come with virtually nothing, so there has to be food," the source told Reuters news agency.

"So this is now a huge concern - where is this food coming from for at least the elderly, the children and the women who have come over without their husbands?

"After the president’s conversation with his Myanmar counterpart ... permission was given for 1,000 tonnes of aid to be sent initially," Ibrahim Kalin, Erdogan spokesman, said in a statement.

"At the first stage, 100,000 [Rohingya] families on both sides of the [Myanmar-Bangladesh] border will receive aid."

Kalin said military helicopters would be used because of concerns over safety.