UN Secretary-general Ban Ki- moon on Thursday strongly encouraged all heads of state and government to accept the invitation issued by Danish Prime Minister Lars Lokke Rasmussen to attend the summit-level gathering on Dec. 17 to 18 at the United Nations climate change conference in Copenhagen.

"The Secretary-general believes that direct head of state and government involvement is essential for governments to reach agreement on the core issues at the heart of a global climate change deal," Ban's spokesperson said in a statement.

The conference, set to begin on Dec. 7, aims to reach accord on a successor to the Kyoto Protocol, the 1997 pact -- part of a larger UN climate change treaty -- which has strong, legally binding measures committing 37 industrialized states to cutting greenhouse gas emissions by an average of 5 percent against 1990 levels over the period from 2008 to 2012.

It will culminate in a summit-level gathering on Dec. 17 and 18, which Ban encourages all world leaders to attend.

"The Secretary-general believes it is essential to maintain political momentum at the highest level and from all sectors of society, and is optimistic that an ambitious, fair and effective climate deal can be reached at Copenhagen," the statement said.