* Chavez has been fighting pelvic region cancer since 2011
* Chavez treated in Cuba, returned to Venezuela on Monday
* Morales stopped in Caracas on Tuesday on way to New York
By Michelle Nichols
UNITED NATIONS, Feb 20 (Reuters) - Bolivian President Evo Morales said on Wednesday he was unable to see his friend, Venezuelan President Hugo Chavez, when he stopped in Caracas on his way to the United Nations in New York because the socialist leader was still being treated for cancer.
Morales, who was in Caracas on Tuesday, said he spoke with Chavez's family and doctor and described the socialist leader's surprise return to Venezuela on Monday - more than two months after undergoing cancer surgery in Cuba - as a "great relief."
"My understanding is that they are very encouraged," Morales told a news conference at the United Nations, where he took part in the global launch of International Year of the Quinoa. "But sometimes diseases, illnesses are difficult to fight."
"He's gone through the most difficult moments in his life," he said. "There are days where the situation of his health is very difficult, according to the information from his ministers. But now he's returned to Caracas and that's a great relief."
Chavez, who has ruled Venezuela for 14 years, has not spoken in public since his Dec. 11 operation in Cuba. He was re-elected for a new six-year term in October but was too ill to return from Cuba for a Jan. 10 inauguration, which was postponed.
Chavez is battling cancer in the pelvic region discovered in June 2011 by Cuban doctors and has undergone four cancer-related surgeries on the communist island.
The first photos of Chavez since his December surgery were published on Friday and showed the 58-year-old in a Cuban hospital breathing through a tracheal tube. Officials said he was now being treated at a Caracas military hospital.
"He's resting; he's still under treatment," said Morales of his fellow leftist. "I hope President Chavez will soon, once again, be at helm of Bolivarian Revolution in Venezuela, as always serving his people."
Morales, who took office in Bolivia in 2006, said he was pained by Chavez's health problems and also by the fact that Fidel Castro was no longer president of socialist ally Cuba.
Castro, who ruled Cuba for 49 years after taking power in a 1959 revolution, has been battling his own health problems since undergoing emergency surgery for intestinal bleeding in 2006. He resigned the presidency in February 2008.
"I remember both of them (Chavez and Castro) told me 'Evo, you have to look after yourself; you have to rest.' They were telling me to rest and I didn't. I wasn't listening to them. And now I do see they weren't resting either," Morales said. (Reporting by Michelle Nichols; Editing by Cynthia Osterman)