ENTEBBE, Uganda (Reuters) - The killings and other atrocities going on South Sudan amount to a genocide and African leaders need to “step up” and not just rely on others for a response, Britain’s secretary for international development, Priti Patel, said late on Wednesday.

After a visit to Africa’s youngest nation, Patel also told journalists in neighboring Uganda that President Salva Kiir’s government was blocking access to aid.

“There are massacres taking place, people’s throats are being slit ... villages are being burnt out, there’s a scorched-earth policy,” she said.

“It is tribal, it is absolutely tribal, so on that basis it is genocide.”

South Sudan has been convulsed by mass violence since July when fighting broke out in the capital Juba, and then spread to several other areas.

The fighting erupted between forces loyal to Kiir and his former vice president, Riek Machar, who come from rival ethnic groups.

Civilians who have fled the violence to neighboring countries say government troops, mostly drown from Kiir’s Dinka tribe, carry out killings and other crimes against Machar’s Nuer and other smaller tribes suspected of supporting rebels.

The United Nations estimates about 3 million South Sudanese have been uprooted by the violence, the biggest cross-border exodus in Africa since the 1994 Rwandan genocide.

More than half of those have fled to neighboring countries, mostly to Uganda.

The mass exodus and widespread insecurity across South Sudan have left farm fields deserted which has led to severe food shortages.

In February, the United Nations declared famine in some parts of the country.

Patel, who also visited refugee camps in northwestern Uganda hosting South Sudan refugees, said Kiir’s government had “actively blocked and prevented aid access” and that it was using food as a weapon of war.

She said she had told Kiir in a meeting with him on Tuesday that she expected him to act to stop blocking aid and also end the conflict.

In case Kiir refused, she said: “The international community will undertake consequences.”

Patel also criticized African leaders for not putting pressure on the South Sudanese government to end the atrocities and conflict and accused them of looking to others to solve a conflict in their backyard.

“Why are they not standing up for the people that are being massacred ... who are their fellow African brothers and sisters,” she said.

“African heads of state ... they need to do a lot more and they should not just rely on others in the international community.”

The July 2016 fighting exploded just as the oil-producing country was still reeling from its first civil war which broke out in 2013 after Kiir sacked Machar from his vice presidency position.

That conflict ended in a peace pact in 2015 and Machar was returned to his vice presidency position early last year. But tensions between the two men lingered and finally led to the latest violence.