Iraq's Kurdish region claimed disputed northern oilfields on Friday in another blow to efforts to forge a united front against a jihadist-led onslaught, as twin bombings killed 28 people.

The escalating row between Baghdad and the Kurds and the blasts in disputed Kirkuk province, which killed mostly refugees from fighting elsewhere, came just two days before a planned parliamentary session to revive flagging efforts to form a new government.

With political disputes deepening and militants on the attack, the US embassy in Baghdad issued a sharp rebuke to Iraqi politicians, saying any additional delays or escalation would only play into the insurgents' hands.

The regional Kurdish government further raised the stakes Friday by claiming control over disputed northern oil fields.

"Production at the new fields under (Kurdish) control will be used primarily to fill the shortage of refined products in the domestic market," it said, adding that staff from the federal North Oil Company could either cooperate with new management or leave.

The move enraged the federal government, which labelled it "irresponsible behaviour which violates the constitution and the national wealth, and disregards the federal authorities and threatens national unity."

The oilfield takeovers come after Kurdish peshmerga fighters moved into stretches of disputed northern areas vacated by Iraqi forces during the initial militant offensive last month, and regional president Massud Barzani has said they will stay there.

But in a sign of the major security challenges Kurdish forces face, a suicide bombing followed by a roadside bomb blast at the entrance to Kirkuk city, the province's capital, killed 28 people and wounded 25 on Friday, health official Sabah Mohammad Amin said.

Many of those killed were people who had fled fighting in neighbouring provinces and were trying to reach safer areas in southern Iraq, a senior security official said.

Prime Minister Nuri al-Maliki has meanwhile accused Barzani of exploiting the chaos created by the offensive led by the jihadist Islamic State (IS), and accused the Kurdish region of harbouring militants.

That claim drew derision from Barzani's office, which shot back Thursday that Maliki has "become hysterical and has lost his balance".

Saying Maliki had "destroyed the country," it demanded he "apologise to the Iraqi people and step down".

Control over Kirkuk and its oil wealth would be the realisation of a long-held Kurdish dream, and Barzani's announcement this month that a referendum on independence was in the works has infuriated the Shiite Arab premier.

- Fighting near Ramadi -

The escalating war of words between Maliki and the Kurds has already cast a pall over the parliamentary session slated for Sunday, and Kurdish ministers said Maliki's stance "only served the enemies of Iraq and the terrorists" and announced they were boycotting cabinet sessions.

Deputy Prime Minister Hussein al-Shahristani has stepped in as acting foreign minister in place of Hoshyar Zebari, a long-serving Kurdish minister boycotting the cabinet, a senior official said.

As the Baghdad-Kurdish row deepened, Sunni militants captured multiple areas west of Anbar provincial capital Ramadi in fighting that began Thursday afternoon, killing 11 police, bombing a police station and capturing another, an officer and a doctor said.

The officer, a police lieutenant colonel, said the insurgents were attempting to "storm Ramadi from the western side".

Army and police officers later said the fighting wound down with the militants in control of new areas but still short of the city.

The fall of Ramadi, where anti-government fighters have held shifting areas since early this year, would be a major advance for the jihadist-led militants who have overrun large areas of five provinces, including parts of Anbar, since June 9.

It could increase the threat to the capital by solidifying militant positions in Anbar and breaking the isolation of insurgent-held Fallujah, only a short drive west of Baghdad.

In his latest rebuke of Iraqi politicians, top Shiite cleric Grand Ayatollah Ali al-Sistani, who is revered by millions, on Friday called for them to stop quarrelling and avoid further delays in picking a new leadership.

The new MPs' first attempt at selecting a parliament speaker on July 1 ended in disarray, with deputies trading threats and some eventually walking out.

The American embassy said Friday that politicians must make progress, and that "further delays or escalation by any side, no matter the pretext, cannot be justified and would disserve the Iraqi people."