Vice President Biden, his wife Jill Biden (second from right) and granddaughter Naomi Biden (third from right) visit the Ottoman-era Sultanahmet mosque in Istanbul on Friday. (Sedat Suna/AFP/Getty Images)

Vice President Biden urged “a change of attitude” by the Turkish government toward its domestic critics Friday, saying that the media and all others here must be free to “challenge orthodoxy,” including political and religious beliefs, if Turkey is to thrive.

Biden’s two-day visit is primarily to meet with top government officials on Syria and other regional crises. But he began with sharp criticism of President Recep Tayyip Erdogan’s crackdown on journalists, political opponents and academics, including the recent arrest of dozens of academics who questioned government violence in Turkey’s ethnic Kurdish region.

Speaking at the beginning of a private meeting with Turkish journalists and civil society representatives, Biden praised U.S.-

Turkish cooperation. But “when the media are intimidated or imprisoned for critical reporting, when Internet freedom is curtailed and social media sites . . . shut down, when more than 1,000 academics are accused of treason simply by signing a petition,” he said, “that’s not the kind of example that needs to be set in the region.”

[Why Turkey’s government is threatening academic freedom]

Critics at home have accused the Obama administration of downplaying human rights abuses here as it seeks Turkey’s full cooperation in the fight against the Islamic State. Biden also defended U.S. Ambassador to Turkey John Bass, who has come under official fire here for speaking in support of the petitioners. “He speaks for my . . . government, he speaks for the American people,” Biden said of Bass.

During an earlier meeting with Turkish lawmakers from a range of political parties, Biden said that the United States supported Turkey’s fight against militants of the Kurdistan Workers’ Party, or PKK, but noted that the Turkish government’s policy of using force against militant-allied villages in the southeastern Kurdish region was not sustainable.

The Turkish government did not publicly respond to Biden’s remarks. Speaking separately to reporters in Istanbul, Erdogan said that the PKK — as well as Syrian Kurds who are fighting with U.S. help against the Islamic State in Syria — is no different from the Islamic State.

In addition to complicating U.S. relations with Turkey, the Kurdish issue is one of several obstacles that may lead to the cancellation or postponement of a Monday meeting in Geneva between Syrian opposition forces and representatives of the government of President Bashar ­al-Assad.

The meeting is part of a process designed by the United States and its European and regional partners supporting the opposition, as well as key Assad backers Russia and Iran, to end Syria’s domestic conflict and allow the parties to join forces against the Islamic State in both Syria and Iraq.

“There are a lot of front-burner national security” issues for which “Turkey is at the heart of our ability to get things done . . . including Iraq, Syria, Cyprus and energy in the eastern Mediterranean,” said a senior administration official, speaking on the condition of anonymity about Biden’s upcoming meetings with Erdogan and Turkish Prime Minister Ahmet Davutoglu.

[17 things Erdogan has said about journalists]

Turkey charges that Syrian Kurdish fighters, known as the People’s Protection Units, or YPG, share the PKK’s goal of establishing a separate political entity in Kurdish regions of Turkey, Syria and Iraq.

To add to the complexity, Turkey has sent troops into northern Iraq near PKK havens, leading Iraqi Prime Minister Haider al-Abadi to threaten to invite Russian planes to strike in the region. Turkish-Russian relations have been strained since Turkey shot down a Russian jet that strayed into its territory from Syria in November.

To assuage Turkey’s concerns about the Syrian border, the Obama administration has pledged that the Syrian Kurdish fighters it supports will not move west beyond the Euphrates River. To secure the remaining 60 miles of border contested with the Islamic State in northwest Syria, Biden plans to firm up U.S. offers of additional equipment and intelligence assistance.

He also hopes to obtain more Turkish cooperation in assembling groups of vetted Syrian opposition fighters — most of whom are currently fighting against Assad — to occupy the remaining border region. That goal has been complicated by pro-Assad Russian airstrikes against opposition forces in the area.

In statements Friday, one of the chief opposition negotiators, George Sabra, said that there would be no Geneva talks unless the airstrikes stop, Syrian government blockades of populated areas are lifted and opposition detainees are released.

Meanwhile, Russia has said it wants the Syrian Kurds added to the opposition negotiating team, and it has rejected attendance

by Islamist opposition groups backed by Turkey.

Biden and his wife also visited the 17th-century Sultanahmet Mosque, also known as the Blue Mosque, and walked from there to a nearby square to lay red carnations on the site where a suicide bomber killed 10 German tourists a little more than a week ago.

As a soft snow fell on Istanbul, they then toured a covered shopping area, where the vice president bought his wife a necklace after Jill Biden and a Secret Service agent persuaded the shopkeeper to lower his price from $350 to $100.