Raila Odinga said Kenyan forces would get to Kismayo by August and said beating al-Shabab would need “an operation by land, sea and air”.
But EU foreign policy spokesman Michael Mann said this went beyond its mandate.
EU warships have been in the area since 2008 to tackle pirates.
Its mandate was recently expanded to allow for land attacks against Somalia-based pirates.
Up to 10 warships are on patrol off the Horn of Africa as part of the EU’s Atalanta operation, which was launched to protect commercial shipping against pirate attacks.
But Mr Mann told the BBC: “We have to be very careful to operate within the bounds of international law. We have our mandate for our mission, which is specifically for fighting pirates at sea and for destroying their armaments dumps.
“If we were going to change it, we would need 27 member countries to agree to that. At the moment we feel fighting al-Shabab is more effectively done through other means.”
African Union soldiers, Ethiopian forces and Somali government troops have in recent months succeeded in driving al-Shabab from several towns.
Kenyan forces moved into Somalia last October and have been slowly moving toward Kismayo.
Kismayo is considered strategically important because the al-Shabab militants who control it get a significant portion of their funds by levying “taxes” on the port.
“Without controlling Kismayo, it’s very difficult to completely neutralise al-Shabab,” Mr Odinga told journalists in Nairobi.
“It has taken time because our forces felt that to move in otherwise would have cost a lot of lives, both civilian and military.”
Mr Odinga said he intended to ask the US to help pay for the assault.
He said the US had previously “resisted” providing funds until Kenyan forces joined the African Union force known as Amisom. They joined earlier this month.
“If they can also bring military assistance so much the better, but for now we are talking about financial assistance,” he said.
He said the EU was “reluctant” to commit because Kismayo is an al-Shabab stronghold and not a pirate stronghold.
But he said the international community should work together to “create an environment in Somalia that will facilitate the voluntary return of Somalis to Somalia.”