A slew of gun attacks in Pakistan's biggest city killed at least 34 people, officials said Thursday, further rattling a country already tense over a military offensive against Taliban militants in a district near the capital. The unrest came as President Barack Obama said he was "gravely concerned" about Pakistan's stability and described its government as "very fragile" - although he did express confidence that the country's nuclear arsenal was safe from militants. Pakistan's president urged the public to support the army's offensive against Taliban fighters so that Pakistan would remain "a moderate, modern and democratic state." Ethnic tension was the suspected spark for the gun attacks Wednesday in Karachi, a teeming southern port city with a volatile history. Much of the tension has been between the Pashtun population, who dominate the country's militant-infested northwest, and Urdu-speakers descended from migrants from India.