The US said Wednesday it is committed to its "longstanding relationship" with Egypt as it continues to voice alarm about the climate of freedom in the county.

A State Department official said Washington was concerned about "polarisation in Egypt" following the army's ouster of Islamist president Mohamed Morsi and a week ahead of a referendum on a new constitution, citing worries voiced by the US-based Carter Centre on Monday.

"We share their concerns about polarisation in Egypt, and have continued to urge Egypt — the government of Egypt — to seriously consider the recommendations of the centre regarding the process of the referendum," spokesperson Jen Psaki said in a Wednesday press briefing.

The Carter Centre, led by former US President Jimmy Carter, had voiced alarm about the "polarised environment and the narrowed political space" in Egypt and the "lack of an inclusive process for drafting and publicly debating" the new charter.

Psaki said that the US remians deeply concerned about the "current climate for freedom of assembly and expression in Egypt."

These include "putting [political] pressure on some human rights organisations and the continuing arrests of citizens for violating the demonstrations law and expressing views about laws."

Authorities have mounted a sustained crackdown on Morsi's Muslim Brotherhood movement in which hundreds have been killed and thousands others, including the group's upper echelons, arrested.

A new protest law passed in November, which rights groups slammed as "repressive," has allowed authorities to detain several non-Islamist activists.

Psaki also reiterated concerns about reports that individuals were arrested allegedly for campaigning for a no-vote on the referendum.

"The quality of the campaign for the referendum, in our view, will affect the credibility of the outcome.," Psaki said.

"The government must permit an open campaign process to allow Egyptians to choose and advocate for a yes or no vote or to abstain from voting."

The upcoming constitutional vote will mark the first watershed of a transition roadmap unveiled by interim authorities after the army deposed Morsi in July following mass protests against his troubled year in power.

The poll will be followed by parliamentary elections and a presidential poll, with officials saying the state is more likely to call a presidential vote first, tweaking an original timetable which had parliamentary elections first.

The US has repeatedly voiced concerns about the designation of the Brotherhood as a "terrorist" group last month.

The Islamist movement was declared a terrorist group after 16 people were killed in a suicide bombing on a police headquarters north of Cairo late December, despite denials of involvement by the Muslim Brotherhood.

Responsibility for the attack was claimed by a  Sinai-based militant Islamist faction.

"We don’t feel these steps move Egypt’s transition forward, and we continue to urge the government to move towards an inclusive, stable, and peaceful path as they move towards a democratic transition," Psaki said.

A US official said following the designation of the Muslim Brotherhood as terrorist that the administration of President Barack Obama was not considering, or even discussing, the possibility of the US government designating the Brotherhood as a terrorist organisation.