Mexico City (AFP) - Mexico aims to extradite drug lord Joaquin "El Chapo" Guzman to the United States by February, a senior official said Friday, but his lawyer promised to "fight to the end."

"We hope (to do it) in January or February," National Security Commissioner Renato Sales Heredia told TV network Televisa, giving Mexico's clearest estimate yet of when it could send the imprisoned Sinaloa cartel chief across the border.

Guzman's lawyer Jose Refugio Rodriguez countered: "He can have his own personal opinion. I think it will be very difficult for it to happen between now and January."

Refugio Rodriguez told Televisa that his client has instructed him to "fight to the end" and that he can win the case if it "is not handled politically."

The foreign ministry approved Guzman's extradition in May, but the man who was once considered the world's most powerful drug trafficker has filed court injunctions.

A judge began reviewing the case on September 26 and it is unclear when he will issue his ruling. Guzman can appeal a decision against him in a higher court.

Guzman is facing two extradition bids, one in California for drug distribution and another in Texas on charges that include murder and money laundering.

- Sons deny ambush -

His sons, meanwhile, have made waves, with authorities accusing them of being behind an ambush on a military convoy last month that killed five soldiers in the northwestern state of Sinaloa.

Refugio Rodriguez gave Televisa a letter from Archivaldo Ivan and Jesus Alfredo Guzman dated October 2 in which they deny their involvement.

"We have nothing to do with the massacre against military personnel," says the letter from the sons, both of whom have been sanctioned by the US Treasury Department as members of the Sinaloa cartel.

"We have never fought with the government, nor do we want to. This would practically mean digging our own graves."

With Guzman in prison since January, his cartel has faced attacks from rivals.

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Members of the Jalisco New Generation drug cartel, which is battling Guzman's gang for supremacy in Mexico, briefly kidnapped Jesus Alfredo in August, according to US and Mexican officials.

He was released within days and the motive is unknown.

Officials say Guzman's criminal group is also being challenged in its northwestern home state of Sinaloa by the Beltran Leyva drug cartel.

- Major US trial -

Guzman's extradition would set up a major trial in the United States for the head of a cartel accused of providing tons of drugs to addicts in the United States while fueling violence in Mexico.

The government's timing for the extradition "may be a bit optimistic but it is not out of the realm of possibilities," Alejandro Hope, a security analyst and El Universal newspaper columnist, told AFP.

"He will obviously defend himself with everything he's got."

Guzman was captured in February 2014 after 13 years on the lam, but he escaped a year later from a maximum-security prison near Mexico City through a 1.5-kilometer (one-mile) tunnel, humiliating President Enrique Pena Nieto.

After he was recaptured in January in Sinaloa, Pena Nieto demanded his speedy extradition.

The president had balked at extraditing Guzman before his July 2015 escape, preferring to put him on trial in Mexico.

Guzman is now imprisoned in Ciudad Juarez, a city bordering Texas.