The UN children's agency says boys and girls as young as seven years old have been abducted, raped and killed in South Sudan.

Dozens of children have been killed, at least 12 raped and others abducted and recruited in attacks in the country's Unity state over a two-week period, according to UNICEF.

Witnesses believe the attacks were carried out by armed groups aligned with South Sudan's military, UNICEF said in a statement on Monday.

"Survivors reported to UNICEF staff that whole villages were burned to the ground by armed groups, while large numbers of girls and women were taken outside to be raped and killed - including children as young as seven," the statement said.

"At least 19 boys, some as young as 10 years of age, and seven girls were killed. Others were mutilated or recruited to join the fighting and take care of stolen cattle."

'A very dark time'

James Elder, a UNICEF spokesperson, told Al Jazeera that "unfortunately these are the numbers we have verified and we know the numbers are on the low side".

"It's quite horrifying and a very dark time," he said.

South Sudan's military recently launched an offensive against anti-government forces, drawing concern from watchdog groups and the regional monitoring bloc over fighting in Bentiu, the capital of Unity state.

Fighting is also taking place in Upper Nile state.

It was not possible to immediately get a comment from South Sudan's military.

Doctors Without Borders reported on Sunday that health workers had fled with their patients into swamps as clashes hit the opposition stronghold of Leer in Unity state.

The International Committee of the Red Cross (ICRC) said on Monday that tens of thousands of people forced to flee the fighting would suffer from a lack of food and health care while on the run.

"How many times will South Sudanese civilians be forced to flee a town under attack, knowing that if they don't their lives will be in danger?" Franz Rauchenstein, head of the ICRC delegation in South Sudan, said in a statement.

"We again ask of those involved in the fighting to not target civilians and let those trying to escape hostilities travel unimpeded."

Refinery capture claim

In another South Sudan-related development, fighters said on Tuesday they had captured a refinery near an oilfield in Upper Nile state.

The anti-government group said all oil companies operating near the Paloich oil fields should shut down and evacuate their staff immediately.

Last month the fighters pledged to capture key oil installations to force President Salva Kiir to step down. Oil extraction is the main source of revenue for the South Sudanese military.

South Sudan's current conflict began in December 2013 as forces loyal to Kiir tried to put down an uprising led by his former deputy, Riek Machar.

Peace talks collapsed in March this year.