JERUSALEM, April 15 (Reuters) - Israel responded to Palestinian rocket fire with two air strikes in the Gaza Strip on Friday, officials said, ending a lull in violence since an informal ceasefire took effect earlier in the week.

Two rockets were fired from the Hamas-controlled Gaza Strip in the direction of the southern Israeli cities of Ashdod and Ashkelon. Neither caused any injuries or damage, an Israeli police spokesman said.

Israel responded hours later with two air strikes against Hamas targets in the coastal territory, Hamas and the Israeli military said. No one was injured in the strikes.

Violence flared last week after Hamas militants fired an anti-tank rocket across the border at an Israeli school bus, critically wounding an Israeli teenager. Israel retaliated with air and ground strikes, killing 19 Palestinians.

Militants in Gaza had fired at least 140 rockets at Israel during the four days of fighting, which had subsided since Egyptian and UN mediators achieved an informal truce on Sunday.

(Reporting by Ari Rabinovitch and Nidal al-Mughrabi; editing by Michael Roddy)