The intensity of the violence added to the sense that insurgent and sectarian attacks had been on the rise in recent weeks.

Bush administration officials have said repeatedly that an increase in troop levels has reduced violence to the point that political and sectarian reconciliation is becoming a reality. The administration has withdrawn some of the reinforcements and is assessing the effect of that reduction before withdrawing additional troops, a decision expected in the next week.

Recent statistics compiled by the Pentagon suggest that after dropping significantly last fall, the number of daily attacks remained static from November through January, the last month for which official figures were available. And the relative calm has been pierced by a flare-up of violence in recent weeks.

No Americans were killed in the shelling on Sunday, officials said, but mortar shells that fell short of their target killed 13 Iraqis in neighborhoods east of the Green Zone. The first attack, about 6 a.m., sent thunderous booms echoing across the city, shaking buildings and rattling windows.

Image A Baghdad resident outside a morgue Sunday after attackers also struck in the Zafaraniya neighborhood. Credit... Ahmad Al-Rubaye/Agence France-Presse — Getty Images

Although the source of the attacks could not be determined conclusively, two witnesses said the early-morning rounds were fired across the river from the Shiite-dominated Baladiyat neighborhood by militia men who the witnesses believed belonged to the Mahdi Army of the Shiite cleric Moktada al-Sadr.