Packages still at 14.04 version after upgrade to 16.04

I've recently upgraded a couple of VPS from 14.04 LTS to 16.04 LTS. The upgrade has apparently ended successfully.

Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.9.36-x86_64-linode85 x86_64)

I've now tried to install php7.0-dev, but it seems like my apt-get tries to install old packages thinking they're at the newest version:

The following packages have unmet dependencies: php7.0-dev :
Depends: libpcre3-dev but it is not going to be installed
          Depends: php7.0-common (= 7.0.22-0ubuntu0.16.04.1) but 7.0.23-1+ubuntu14.04.1+deb.sury.org+1 is to be installed
          Recommends: dh-php but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I have tried to run apt-get update/upgrade/dist-upgrade already. The sources.list file refers to xenial.

What is the problem here? How to download new packages?