UN leader Ban Ki-moon is alarmed by the "dramatic escalation" of the Syrian civil war highlighted by a deadly air attack on a Palestinian refugee camp, his spokesman said on Sunday.

Ban warned that attacks on civilians in the 21-month-old conflict could be treated as "war crimes", said UN spokesman Martin Nesirky.

Syrian army planes attacked the Yarmuk refugee camp in Damascus as government forces stepped up their offensive against rebels south of the capital. Activists said at least eight people were killed.

"The secretary general is alarmed by the continued dramatic escalation of violence in Syria over the past several days, and the grave danger facing civilians in areas under fire," said Nesirky.

"Targeting civilians or carrying out military operations in populated areas, in an indiscriminate or disproportionate fashion that harms civilians is a war crime," the spokesman added.