Mogadishu - The Somalia-based al-Shabaab claims that its escalation of deadly attacks is in "doubled response" to President Donald Trump's approval of expanded US military efforts against it, an organization that monitors extremist groups says.Al-Shabaab's statements come after Somalia's new military chief survived a suicide car bomb attack on Sunday following his swearing-in with a mandate to launch a new offensive against the Islamic extremists. Thirteen people were killed. A day later, a suicide bombing at a military academy in the capital, Mogadishu, killed at least five soldiers.Al-Shabaab's Shahada News Agency posted the group's statement on social media on Monday, the SITE Intelligence Group said.The attacks also are in response to President Mohamed Abdullahi Mohamed's declaration last week of the new offensive against the extremists, al-Shabaab said. The Somali-American president has declared the long-chaotic country a war zone.Mohamed's election in February and Trump's taking power in January have brought the new military efforts against al-Shabaab, which was chased out of Mogadishu several years ago by national and African Union multinational forces but still controls some rural areas.It continues to carry out deadly attacks in the capital, targeting hotels, military facilities and the presidential palace.With Trump's approval, the US military now can pursue more aggressive airstrikes against al-Shabaab, and parts of southern Somalia will be considered areas of active hostilities. US special operations forces can move closer to the fight while increasing assistance to Somali forces.Al-Shabaab has denounced Somalia's new leader as an "apostate" and vowed to further destabilize the country. Al-Shabaab was kicked out of Mogadishu under Mohamed's brief term as prime minister in 2010-2011.Mohamed has vowed to make security a priority in Somalia, where the fragile central government is trying to extend its control beyond selected areas including the capital.