Nuland said U.S. officials were "working closely with the Turkish national police to make a full assessment of the damage and the casualties, and to begin an investigation."
Carney, the White House spokesman, said the attack would strengthen the resolve of Turkey and the U.S.
"Turkey remains one of our strongest partners in the region, a NATO ally," he said. "We have worked shoulder-to-shoulder with the Turks to counter terror threats. Turkey has been a very important ally, broadly speaking and in the effort to counter terrorism."
Foreign Minister Ahmet Davutoglu vowed Turkey would spare no effort in protecting diplomatic facilities.
"We have always shown great sensitivity to the protection of foreign missions and we will continue to do so," he said.
The injured journalist was 38-year-old Didem Tuncay, who until recently had worked for NTV television. A hospital official said she was "not in critical condition."
Ricciardone visited her in the hospital and told reporters outside that he had invited her to the U.S. Embassy for tea.
He also paid tribute to the Turkish guard who was killed, calling him a "Turkish hero" who died while defending U.S. and Turkish staff.
Americans in Turkey were warned to avoid visiting the embassy or U.S. consulates in Istanbul and Adana until further notice and were told to register on the State Department's website.
"The Department of State advises U.S. citizens traveling or residing in Turkey to be alert to the potential for violence, to avoid those areas where disturbances have occurred, and to avoid demonstrations and large gatherings," the U.S. Consulate in Istanbul said in a statement.
British Foreign Secretary William Hague condemned the attack "in the strongest terms" and said Turkey and the U.S. will get the U.K.'s full support as they seek to hold those responsible to account.
NATO Secretary-General Anders Fogh Rasmusen added his condemnation, calling it "an outrageous attack" that "shows a reckless disregard for human life and for the inviolability of diplomatic staff."
Israeli Prime Minister Benjamin Netanyahu sent a message to President Barack Obama, saying he was "shocked and saddened to learn of the vicious terrorist attack."
Ed Royce, the chairman of the U.S. House Foreign Affairs Committee, said the attack was "another stark reminder of the constant terrorist threat against U.S. facilities, personnel and interests abroad."
"Coming after Benghazi, it underscores the need for a comprehensive review of security at our diplomatic posts. The committee stands ready to assist the State Department in protecting our diplomats," he said in a statement.
Turkey's parliament speaker, Cemil Cicek, linked Friday's attack to the arrest last month of nine Turkish human rights lawyers, who prosecutors have accused of links to the DHKP-C.
"There was an operation against this organization," Cicek said and suggested the attack could be an attempt by the group to "say 'We are still here, we are alive.'"
James F. Jeffrey, a visiting fellow at the Washington Institute for Near East Policy who was U.S. ambassador in Turkey between 2008 and 2010, said DHKP-C was a resilient group that had been "relatively quiescent" in recent years. He said the organization was born out of the 1970s European tradition of pro-communist terrorism, and he drew a parallel with Germany's now-defunct Baader-Meinhof gang.
"I do not see them as a major threat compared to al-Qaida," Jeffrey said of DHKP-C in a conference call with journalists. The group, he said, typically attacks with small arms and conducts periodic assaults "just to make sure people know they're still out there."
He said it seemed to have "very deep roots" and means of recruitment in several urban centers, including Istanbul, Ankara and possibly the coastal city of Izmir. Jeffrey said it was unlikely the attack was a response to recent regional developments — including, for example, Israel's strike this week on a Syrian target — but did not rule out that DHKP-C conducted the bombing as a kind of subcontractor for another group.
He also said the embassy's strong defenses worked as they were supposed to, with the "minimum number of casualties" for such a grave attack.
"It's a very hard perimeter to crack, as we saw today," Jeffrey said.
In past years, the DHKP-C group has spearheaded hunger strikes against Turkish prison conditions that led to the deaths of dozens of inmates. The protesters opposed a maximum-security system in which prisoners were held in small cells instead of large wards.
In September, police said a leftist militant threw a grenade and then blew himself up outside a police station in Istanbul, killing a police officer and injuring seven others. Police identified the bomber as a member of the DHKP-C.
In 2008, Turkish police said they had foiled a bomb plot by DHKP-C against some U.S. companies in Turkey.
Turkey has also seen attacks linked to homegrown Islamic militants tied to al-Qaida. In a 2003 attack on the British consulate in Istanbul, a suspected Islamic militant rammed an explosive-laden pickup truck into the main gate, killing 58 people, including the British consul-general.
Associated Press writers Ezgi Akin in Ankara, Turkey, Christopher Torchia in Istanbul, Nedra Pickler and Bradley Klapper in Washington, Raf Casert in Brussels, and Cassandra Vinograd in London contributed to this report.