Three back-to-back explosions hit India's financial capital Mumbai Wednesday, killing at least 21 people and wounding more than 100 others in what authorities are calling a “coordinated attack by terrorists.”

The blasts occurred in crowded areas of the city Wednesday evening — two in the south and one in central Mumbai. The special secretary for internal security, U.K. Bansal, said that it appeared to be a terrorist attack “intended to cause casualties.”

No one has immediately claimed responsibility for the explosions. Authorities are investigating.

Wednesday's violence is the worst to hit Mumbai since Pakistan-based militants laid siege to the city in 2008, killing 166 people.

Pakistan's government swiftly condemned Wednesday's bombings.

U.S. President Barack Obama also issued a statement strongly condemning what he called the “outrageous attacks” in Mumbai and expressing his condolences. He pledged that the American people would stand by their close friend and partner and offer support to India's efforts to bring those responsible to justice.

Wednesday's explosions also happened just two days after the fifth anniversary of a series of train bombings in Mumbai that killed more than 180 people. Indian authorities blamed that attack on Pakistani militants.