Heavy explosions and gunfire are shaking South Sudan's capital, Juba, as clashes between government and opposition forces enter their fifth day, witnesses say, pushing the country back toward civil war.

An Associated Press reporter in the city reported shooting Monday morning and preventing residents from moving about the city.

An aid worker in Juba's Tomping area, which has embassies, the airport and a U.N. base, described a "massive explosion" shortly after 9 a.m. followed by further blasts

The United Nations Mission in South Sudan reported on Twitter that clashes restarted near its compound in the Jebel area, where some 30,000 civilians have taken refuge. The opposition also has a base near Jebel and their leader, First Vice President Riek Machar, also has his home in the area.