US President Barack Obama urged Japan's new centre-left government on Tuesday to keep a pact allowing US bases on its territory, as both sides laid the groundwork for his visit to Tokyo this week. Obama also said he wanted to one day become the first sitting US president to visit the nuclear-bombed cities of Hiroshima and Nagasaki, while Japan said it would help the US effort in Afghanistan with five billion dollars in aid. On the sensitive security issues, Obama said he understood Prime Minister Yukio Hatoyama's government wanted to review the two nation's post-war security alliance after it ended five decades of conservative rule in September.