* Reconciliation a "very delicate process" - presidential aide
* Britain says Sri Lanka should conclude independent probe by March
* U.N. urges punishment of soldiers involved in atrocities
WASHINGTON, Jan 27 (Reuters) - An international inquiry into war crimes in Sri Lanka would bring "chaos," and the government's national reconciliation process must be given several more years to work, a top aide to Sri Lanka's president said on Monday.
Lalith Weeratunga, in Washington to lobby against calls for such an inquiry, said Sri Lanka needed at least five years from the July 2012 date the government regards as the start of its reconciliation process for the effort to take root.
"After 26 years of conflict ... we want to make it a sustainable peace. It's a very delicate, delicate process. Reconciliation is not a task that can be achieved in a day or two," said Weeratunga, who is secretary to President Mahinda Rajapaksa and Sri Lanka's top civil servant.
The final few months of Sri Lanka's long civil war in 2009 were by far the bloodiest, and both the government and Liberation Tigers of Tamil Eelam (LTTE) rebels blame each other for the deaths of tens of thousands of mainly Tamil civilians during that period.
The United Nations has called on Sri Lanka to punish those in the military involved in atrocities, and U.S. Embassy officials in Colombo say Washington plans to introduce a resolution calling for an international investigation at the March session of the U.N. Human Rights Council.
British Prime Minister David Cameron said in November he would push for an international inquiry if Sri Lanka did not conclude an independent investigation by March.
U.S. Embassy officials say the United States will demand that the government investigate other abuses as well as alleged war crimes, including lack of progress in reconciliation, abductions of anti-government critics, attacks on churches and mosques and the media, and alleged restrictions on freedom of association and labor union activity.
But Weeratunga rejected the charges and said an international inquiry would only reopen old wounds, as it would require investigation of all those involved in the conflict, including former rebels and Indian peacekeepers as well as the armed forces.
"There would be huge chaos in the country ... armed forces that liberated the country from terrorism would again be put to a judicial test. That is really going to reduce the morale of the army. These are things that need to be considered very carefully," he told Reuters in an interview.
"Why single out Sri Lanka for an international inquiry when ... other countries that have gone through more difficult issues are not be investigated?," he asked.
"If there is an international investigation, the whole period has to be investigated - from the 1980s onward - which includes the two-year tenure of the Indian peacekeeping force, which will upset India, which will upset our relationship with India."
Weeratunga said the government needed more time for its efforts to resolve land disputes, resettle people displaced by war and improve people's livelihoods.
He also said the government was investigating 13,000 cases of missing persons, a task complicated by the unwillingness of countries like Britain to provide information on people thought to be living there.
Ten cases of attacks on churches and mosques were "being dealt with severely," he said, but added it took time to probe cases where there was no evidence as to who carried them out. (Reporting by David Brunnstrom in Washington and Shihar Aneez in Colombo; editing by Peter Cooney and G Crosse)