Before leaving Ankara on 27 July, the Turkish leader said he would try to persuade Iran to list the former PKK -- known now as Kongra-Gel -- as a "terrorist organization."
The Eastern Anatolian-based PKK has led a 15-year armed uprising against Turkey that has claimed an estimated 35,000 lives.
Following the arrest and trial of its leader Abdullah Ocalan in the late 1990s, the group officially laid down weapons and sought refuge in neighboring Iraq and Iran.
After it changed its name to Kongra-Gel, the group in June called off the cease-fire it had declared in 1999, accusing Turkey of not respecting the truce.
Fighting has since been reported almost daily in Turkey's predominantly Kurdish provinces. The latest incident occurred yesterday when militants launched an attack in Diyarbakir, injuring a police officer and the deputy mayor in the southeastern city.
A few hours earlier, Erdogan had indicated in Tehran that Turkey and Iran were nearing an agreement on the joint fight against Kurdish rebels.
In comments made after meeting Iranian First Vice President Mohammad Reza Aref and parliamentary speaker Gholam Ali Haddad-Adel, the Turkish prime minister hinted he had secured Tehran's promise to outlaw the PKK and its successor.
Erdogan was asked by a Turkish reporter whether Iran had formally agreed to list the group as a terrorist organization: "You know that with regard to the issue of terrorism, Iran and Turkey have decided to adopt a common stance. [On 29 July], we will sign a memorandum of understanding pertaining to this issue. This memorandum will be signed in such a way that the Kongra-Gel/PKK terrorist organization will find its place in it with many details."
Iranian officials have not specifically commented on the expected deal. Iran's official IRNA news agency yesterday quoted Aref as simply saying both sides are confident security talks will lead to "good results."
Turkish commentators believe an agreement to join forces against Kurdish fighters would -- at least in theory -- represent a milestone in bilateral ties. In the past, the two countries have blamed each other for sheltering respective Kurdish militants, but have moved recently to improve uneasy relations.
Turkey has long suspected Iran of secretly supporting the PKK.
Tehran, in turn, blames Ankara for allegedly offering shelter to the left-wing Kurdistan Democratic Party of Iran and a rival group known, in short, as Komala.
Yet, both capitals have moved to cooperate on security issues.
Iran earlier in July launched a crackdown on PKK militants holed up along its border with Turkey. Turkey's NTV private television station reported at the time that six Kurdish fighters and 10 Iranian troops were killed in the operation. Tehran officially admitted to the death of 10 Kurdish peshmergas and only two of its soldiers.
What prompted Iran -- which itself faces U.S. accusations of sponsoring terrorism -- to move against the PKK is unclear.
Erdogan yesterday praised Tehran's new stance: "We've seen that Iran favors the creation of a joint platform against terrorism, whatever its origins. With regard [to terrorism], they have paid a heavy price in the past. So have we. We don't want to pay such a price any longer."
Also uncertain is what joint steps Ankara and Tehran could possibly take against left-wing Kurdish militants.
The English-language "Turkish Daily News" yesterday quoted Firuz Dowlatabadi, Iran's ambassador to Ankara, as saying joint military operations would require too much paperwork and organization to be conceivable in the near future.
Meanwhile, some both in Turkey and Iran appear to be questioning Tehran's commitment to defeat the PKK.
Bahram Valadbeigi, who runs the Tehran-based Kurdish Cultural Center, told Radio Farda today that he believes Iran's agenda does not necessarily coincide with Turkey's: "The Islamic Republic of Iran is not completely against the PKK. It has always had some limited and vague relations with it. I don't believe Iran will take part in Turkey's plans."
Reporting on Erdogan's talks with Iran's first vice president, the liberal "Sabah" daily today struck an ironic note: "Whatever Erdogan had to say -- be it on the PKK, Cyprus, or Palestine -- Aref invariably replied: 'We agree'."