As an Exxon executive Rex Tillerson brings deep ties to Russia. | Getty ExxonMobil CEO Tillerson emerging as frontrunner for secretary of state

ExxonMobil president and CEO Rex Tillerson is the leading contender to be America's next secretary of state, according to two Trump transition aides.

Tillerson, an oil executive with extensive experience in international negotiations, has risen the top of Trump's short list for Foggy Bottom in recent days. Former New York City mayor Rudy Giuliani withdrew from the process late Friday afternoon, saying he planned to return to the private sector.

Tillerson was brought into Trump Tower for an interview with Trump at the recommendation of former Secretary of State Condoleezza Rice and Defense Secretary Robert Gates, who count Exxon among their private consulting clients, according to two sources familiar with the conversations. His name was first publicly floated for the job in early December and he met privately with Trump on Tuesday. Rice sat down with the President-elect in late November, and Gates followed her three days later.

While Gates praised Trump's choice of retired Marine General James Mattis to lead the Department of Defense, he has cautioned against the selection of another general to lead the State Department. He said after his meeting with Trump that he thought "it would be awkward to have military officers both [as] secretary of state and secretary of defense."

A transition source said Tillerson "fits the prototype" and that Trump "likes his style and background."

Other contenders said to still be in the mix include Adm. James Stavridis, Foreign Relations Committee chair Sen. Bob Corker (R-Tenn.) and former U.N. ambassador John Bolton, one aide said. Former Ford CEO Alan Mulally also met with Trump recently to discuss the position.

At Exxon, Tillerson has been known for his ability to reach complex international agreements. He also brings deep ties to Russia: As an Exxon executive he managed ties between the company and the Kremlin and in 2011 struck a deal that allowed the company to access Arctic resources in Russia.

But that deal was blocked by subsequent U.S. sanctions against Russia — sanctions that Tillerson sharply criticized for failing to consider the "broad collateral damage" they caused.

Tillerson joined the oil giant as an engineer in 1975, working his way up to run the massive oil firm. If he is nominated, his extensive holdings in a company with massive international assets, as well as his personal ties to Russian President Vladimir Putin, could complicate his confirmation process.

Josh Gerstein and Alex Isenstadt contributed reporting.