Multiple bomb blasts at two bus stations in Gombe, Nigeria, killed 37 people on Wednesday, while two suicide bomb attacks killed at least 13 people in northern Cameroon.
The United States "strongly condemns the horrific and indiscriminate suicide attacks," the State Department said.
"Boko Haram's unconscionable use of children as suicide bombers and indiscriminate targeting of men, women and children highlights the group's senseless brutality," State Department deputy spokesperson Mark Toner said in a statement.
The attacks came as Nigerian President Muhammadu Buhari was on a four-day visit to Washington, where he met with US President Barack Obama. The two leaders on Monday discussed security issues including the threat posed by extremist group Boko Haram.
Following their talks, Obama said Buhari had a "clear agenda" for defeating the militants and tackling corruption.
Boko Haram has killed thousands of people in a six-year insurgency, during which it has attempted to set up a state imposing strict Islamic law in the northeast of Nigeria, Africa's top oil exporter and most populous nation.
Boko Haram has been increasingly employing young people as suicide bombers since it allied itself with Islamic State. In an attack last Friday, a 10-year-old girl blew herself up in Damaturu, Nigeria, killing at least nine people who were worshipping in a praying area for the Eid al-Fitr holiday.
The United States supports counterterrorism efforts in Nigeria with intelligence, advisers, equipment and logistics, the State Department said.
"We remain committed to working closely with Cameroon, Nigeria and the other Lake Chad Basin countries to defeat Boko Haram," Toner said in the statement.
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