Prime Minister Shigeru Ishiba sounded cautious Sunday about expanding the ruling coalition between his Liberal Democratic Party and Komeito.

"A coalition should be formed after a certain level of agreement is reached on diplomacy, security and public finances," Ishiba said in a speech ahead of Thursday's start of the official campaign period for the July 20 election for the House of Councilors, the upper chamber of Parliament.

At a conference hosted by a private policy proposal group, Ishiba also expressed support for the idea of establishing a nonpartisan consultative body on social security reform.