A senior US diplomat has told Burundi's President Pierre Nkurunziza that the east African country risks "boiling over" if it suppresses political opposition, as protests against the president's bid for a third term entered a fifth day. 

Tom Malinowski, US assistant secretary of state for democracy, human rights and labour, arrived in Burundi on Wednesday to try to help halt escalating unrest and resolve the country's biggest crisis in years.

Malinowski said on Thursday he had told Nkurunziza during a meeting with him that the government must allow peaceful criticism and room for political opposition ahead of June 26 elections.

"I left the president with the thought that this country with its very complicated and difficult history is like a boiling pot, and that if you try to put a lid on that pot it doesn't stop boiling. It risks boiling over," Malinowski told reporters after the meeting.

At least six people have been killed since the unrest began five days ago, triggered by the ruling CNDD-FDD's endorsement of Nkurunziza as a candidate for the June elections.

Supporters of Nkurunziza, a former rebel leader who became president in 2005, say that although he has served two five-year terms, he was elected by parliament for his first term and not directly by voters, making him eligible for a third term.

But opposition figures and rights groups say that Nkurunziza's attempt to stand for a third consecutive term goes against the constitution as well as a peace deal that ended a civil war in 2006.

Hundreds of thousands of people were killed in the 13-year conflict, which divided the country along ethnic lines, between the Hutu majority - to which Nkurunziza belongs - and minority Tutsis. 

Protests 'illegal' 

Nkurunziza told Malinowski that protests against him were illegal but that the opposition, who have threatened to boycott the election if the president contests, would not be restricted, according to presidential spokesman Gervais Abayeho.

The president said "political space would be respected and there is no restriction whatsoever to anybody who is engaged in political competition. Everyone has a role to play," Abayeho said.

Al Jazeera Malcolm Webb, reporting from the capital Bujumbura, said a local radio on Thursday reported that a policeman had shot dead a soldier on the street, although he could not independently verify the report.

Soldiers have been monitoring protests and urging police not to use excessive force, he said. 

The Red Cross also said nine protesters were hurt in Bujumbura in renewed clashes.

Those injured included several who suffered gunshot wounds, Burundian Red Cross spokesman Alexis Manirakiza said. A military source confirmed that police had again fired on groups of demonstrators.

Authorities on Thursday closed university accommodation facilities and forced thousands of students to leave the campus, witnesses said, in an operation apparently part of efforts to halt the wave of protests.

Large numbers of students, many of whom come from rural areas, could be seen evacuating the University of Burundi, situated in Bujumbura, after the government order was issued overnight.

The authorities have already cut mobile access to several social networks and messaging applications including Twitter, Facebook and WhatsApp, which have been used to coordinate protests.

Source: Al Jazeera and agencies