He told the UK's Financial Times paper that financial pressures had to be taken into account and said the commitment was not open ended.
The withdrawal of Ethiopians is a key demand of the Islamist insurgents.
Al-Shabab, the radical wing of the Islamists who controlled much of Somalia in 2006, has refused to recognise a recent UN-brokered agreement the interim government has signed with an opposition group including a top Islamist leader.
It has demanded that Ethiopian troops leave Somalia before any ceasefire is considered.
Somalia has experienced almost constant civil conflict since the collapse of Mohamed Siad Barre's regime in January 1991.
"The operation has been extremely expensive so we will have to balance the domestic pressures on the one hand and pressures in Somalia on the other and …
Rep. Dennis Kucinich: “Wake Up America!” : Information Clearing House - ICH Former presidential candidate Rep. Dennis Kucinich delivered one of the most passionate addresses Tuesday night. “Wake up, America. We went into Iraq for oil. The oil companies want more,” Kucinich said. “War against Iran will mean $10-a-gallon gasoline. The oil administration wants to drill more, into your wallet. Wake up, America. Weapons contractors want more. An Iran war will cost 5 to 10 trillion dollars.”