WASHINGTON (REUTERS) - President Barack Obama vowed on Monday that the United States (US) will find out who carried out explosions in Boston and will hold them accountable.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts,"Obama said in a televised statement.

"But make no mistake, we will get to the bottom of this, and we will find out who did this, we'll find out why they did this." Obama said he has directed the federal government to increase security around the US as necessary after the explosions.