The EU aims to cut emissions by 20% from 1990 levels by 2020
President Obama's decision to attend the UN climate talks in Copenhagen has been welcomed by European leaders.
Swedish Environment Minister Andreas Carlgren said Mr Obama's presence next month would raise expectations.
The US earlier announced that President Obama would pledge to cut greenhouse gas emissions in several stages, beginning with a 17% cut by 2020.
However, BBC North America editor Mark Mardell says many environmentalists regard the US targets as disappointing.
'Ambitious global deal'
UN climate chief Yvo de Boer said Mr Obama's presence in Copenhagen would "be critical to a good outcome".
Danish Prime Minister Lars Loekke Rasmussen said that Mr Obama's visit "emphasises the president's will to contribute to an ambitious global deal in Copenhagen".
President Obama will arrive at the summit on 9 December, two days after it opens.
But he will not stay for the end of the 12-day meeting, when delegates are hoping to pull together a deal.
While in Europe he will travel to Oslo to collect his Nobel Peace Prize, officials said.
The UN summit aims to draw up a new treaty to supplant the 1997 Kyoto Protocol, although observers say this is unlikely.
US officials said Washington would pledge a 17% cut in emissions from 2005 levels by 2020, 30% by 2025, 42% by 2030 and 83% by 2050.
However, most other countries' targets are given in comparison with 1990 figures.
BBC environment correspondent Richard Black says that on that basis the US figure amounts to just a few percentage points, as its emissions have risen by about 15% since 1990.
This is much less than the EU's pledge of a 20% cut over the same period, or a 30% cut if there is a global deal; and much less than the 25-40% figure that developing countries are demanding.
Mr Carlgren, whose country holds the rotating EU presidency, said he was pleased the US would deliver an emissions reduction target, but added: "It needs to be sufficiently ambitious.
"An agreement in Copenhagen will stand or fall on sufficiently ambitious targets by the US and China."
Delegations from 192 countries will be attending the summit.
Leaders saying they will attend include UK Prime Minister Gordon Brown, French President Nicolas Sarkozy and Brazilian President Luis Inacio Lula da Silva.
European Commission President Jose Manual Barroso said it was essential for as many world leaders as possible to attend.
Hu Jintao, president of the world's largest polluter, China, is yet to commit to attending.
The US is the second largest polluter after China.