* Tanzania's main international airport re-opened
* Prime minister says casualties may rise
* Previous munitions dump blast in 2009 killed 26
(Adds airport reopens)
By Fumbuka Ng'wanakilala
DAR ES SALAAM, Feb 17 (Reuters) - A series of blasts at a Tanzania military munitions dump killed at least 20 people late on Wednesday, flattening neighbouring buildings and sending thousands of local residents fleeing in panic.
Prime Minister Mizengo Pinda told parliament on Thursday that 4,000 people had sought shelter at the national stadium in the commercial capital Dar es Salaam, where the blasts occurred.
"The death toll might rise when we ascertain the full extent of the damage caused by the explosions," Pinda said in a statement to parliament.
Reuters television footage showed badly wounded victims -- one child bled from facial injuries and another had shrapnel protruding from his lower back. Several others were rushed to local hospitals with broken bones or severed limbs.
Defence Minister Hussein Mwinyi told Reuters the explosions had stopped but officials had yet to determine the cause.
Some 23 munitions depots, a secondary school and two houses were razed in the chain of explosions at the Gongo la Mboto military base in Ilala district.
Tanzania's Civil Aviation Authority told Reuters the main Dar es Salaam International Airport, located about 10 km from the blast site, re-opened around lunchtime.
Aviation officials said there was no damage reported there or at the aiport on the semi-autonomous island of Zanzibar, one of Tanzania's leading tourist destinations, which was temporarily closed overnight.
The blasts scattered debris across densely populated neighbourhoods several kilometres (miles) from the military base.
"My neighbour's house was struck by debris from a rocket propelled grenade. Luckily, no one was injured because we were told to evacuate our homes," said Hassan Kondo, a resident of the Kivule area on the outskirts of Dar es Salaam.
Army officials said they were planning controlled detonations of unexploded bombs later on Thursday afternoon.
"We urge members of the public to be careful not to touch any debris or unexploded bombs at their areas. Lieutenant General Abdulrahman Shimbo, chief of staff of Tanzania's armed forces, told reporters.
"We have sent out teams to carefully retrieve these explosives," he said.
A similar explosion in 2009 at another military base in Dar es Salaam killed at least 26 people and wounded more than 700.
Additional reporting by Nicholaus Mtenga in Dar es Salaam and Wangui Kanina in Nairobi; Editing by George Obulutsa